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2006 Lotus Elise
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i’m not sure why but my NA 2zzge lotus elise with 23,000 miles on it already has code 0420 with the stock cat. all the mechanics i’ve talked to have told me it barely passes emissions if they baby it during testing. they’ve all said to get a new cat but i can’t find one. is the MWR high flow cat ok for emissions? in my state they seem to only test it with an obd2 reader for emissions codes. also i have code 0171 and over 15% LT fuel trims which seems to be a false lean code because nobody can find any vacuum leaks. could they be related and the cat is secretly fine? i’m thinking it’s an exhaust leak where the header and cat meet. I’d love some experience from someone with a MWR cat or any high flow cat
 

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ik high flow cats are pretty useless on NA cars, and trust me i’d rather go catless with a defouler simply because i like the sound. but my car has an eliminator v3 on the back and i removed the heat shielding, so it would be painfully obvious that the cat is missing. literally by simply glancing at it from the back you can see the cat exposed and he would fail my inspection every time. (also i can’t find an OEM cat ANYWHERE online.)
 

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It isn't a false code since your trims are so off - that confirms that the ECU really is dumping fuel at it which can also cause the cat to foul up. Failure of the pre-cat O2 sensor can also cause both codes, and this isn't unlikely with a car that has sat that much - moisture in the exhaust can cause the sensors to fail.
 

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I would be surprised if the cat failed after 23k miles - unless there were a lot of track miles. Track use will kill the cats.

If you need a cat, have a look at our sport cats: HERE
 

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It isn't a false code since your trims are so off - that confirms that the ECU really is dumping fuel at it which can also cause the cat to foul up. Failure of the pre-cat O2 sensor can also cause both codes, and this isn't unlikely with a car that has sat that much - moisture in the exhaust can cause the sensors to fail.
i’ll see if i can get the sensors tested but i thought they needed to pass the readiness test in order to display 171 in the first place?also the car has i think roughly 4,000 track miles from before i owned it. i will say we found 1 issue where a mouse,etc. chewed through the MAF wire and we replaced the pigtail with a toyota corolla’s one and spliced the wires. before we fixed that, the car was bucking and running terribly. i replaced the MAF shortly after that with a denso one but the codes are the same. as far as i can tell the performance isn’t effected, but maybe i just got used to it being slower. it also had a k&n cold air intake that was literally solidly packed full of hard dirt. so i replaced the k&n cone with a spectre cone. not sure if that would effect the expected airflow bc i think the spectre might be bigger. but im cleaning the k&n to put back on. (i don’t have the oem intake)
 

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i’ll see if i can get the sensors tested but i thought they needed to pass the readiness test in order to display 171 in the first place?also the car has i think roughly 4,000 track miles from before i owned it. i will say we found 1 issue where a mouse,etc. chewed through the MAF wire and we replaced the pigtail with a toyota corolla’s one and spliced the wires. before we fixed that, the car was bucking and running terribly. i replaced the MAF shortly after that with a denso one but the codes are the same. as far as i can tell the performance isn’t effected, but maybe i just got used to it being slower. it also had a k&n cold air intake that was literally solidly packed full of hard dirt. so i replaced the k&n cone with a spectre cone. not sure if that would effect the expected airflow bc i think the spectre might be bigger. but im cleaning the k&n to put back on. (i don’t have the oem intake)
The MAF uses a temperature probe to measure airflow, so if your splice changed that resistance at all then it can really screw with the reading. That being said, I have noticed the MAF reference phases out around 3,000-4,000 rpm. If your engine runs the same on either side of that range, then I'd personally not suspect a bad MAF reading. Here's the enable criteria for P0171:
1266520


For fuel trim to be enabled, the pre-cat O2 sensor will have to be warmed up and reading, but that doesn't mean it is working perfectly. Doesn't mean it isn't either. Just something that could potentially explain both at once.
 

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ik high flow cats are pretty useless on NA cars, and trust me i’d rather go catless with a defouler simply because i like the sound. but my car has an eliminator v3 on the back and i removed the heat shielding, so it would be painfully obvious that the cat is missing. literally by simply glancing at it from the back you can see the cat exposed and he would fail my inspection every time. (also i can’t find an OEM cat ANYWHERE online.)
Have you tried posting a WTB? How about Wirewheel?
 

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Discussion Starter #8
Have you tried posting a WTB? How about Wirewheel?
i would but its confusing that the cat is already bad at my mileage. trying to explore other explanations first, and also prevent it from happening again if it is the cat.
 

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The MAF uses a temperature probe to measure airflow, so if your splice changed that resistance at all then it can really screw with the reading. That being said, I have noticed the MAF reference phases out around 3,000-4,000 rpm. If your engine runs the same on either side of that range, then I'd personally not suspect a bad MAF reading. Here's the enable criteria for P0171:
View attachment 1266520

For fuel trim to be enabled, the pre-cat O2 sensor will have to be warmed up and reading, but that doesn't mean it is working perfectly. Doesn't mean it isn't either. Just something that could potentially explain both at once.
i don't know much about resistance and electrical stuff. i believe to change resistance we would have to add length to the wire or make bad connections correct? i think we did make it a little longer than the original by maybe 6-12". i wasn't a big fan of how the guy who spliced it did the splicing. he just made the wires make contact and used "liquid electrical tape" then wrapped them in actual electrical tape and a long plastic wire shroud. i would've used butt connectors and i've thought about switching it out for butt connectors. think that would help?

i will say though, after doing that job it fixed a LOT of the issues i had with the bucking and stalling. it stills idles rough like a typical elise.

could it be possible that the engine dumped extra fuel into the engine via fuel trim while that bucking/stalling was happening with the MAF wire issue, and any of that extra fuel got on the pre-cat o2 sensor and now its dirty?
 

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i don't know much about resistance and electrical stuff. i believe to change resistance we would have to add length to the wire or make bad connections correct? i think we did make it a little longer than the original by maybe 6-12". i wasn't a big fan of how the guy who spliced it did the splicing. he just made the wires make contact and used "liquid electrical tape" then wrapped them in actual electrical tape and a long plastic wire shroud. i would've used butt connectors and i've thought about switching it out for butt connectors. think that would help?

i will say though, after doing that job it fixed a LOT of the issues i had with the bucking and stalling. it stills idles rough like a typical elise.

could it be possible that the engine dumped extra fuel into the engine via fuel trim while that bucking/stalling was happening with the MAF wire issue, and any of that extra fuel got on the pre-cat o2 sensor and now its dirty?
Any connection like that without preload is going to have some resistance. I would've soldered and heatshrunk, but you can give it a sanity check by reading the intake air temperature when the engine is cold and immediately just cranked. This should be the exact same as your coolant temp which should be the exact same as ambient unless your wiring causes a bad reading. It would have to be really bad to cause 15% error though since it is proportional to absolute temperature. So it would have to read -10°F to think you have 15% more air density. Still, easy to check. This also will only be the case if you messed with the wiring for the supply or reference temp.

My Elise doesn't idle rough, for what that's worth. It isn't my BMW I6, but I wouldn't call it "rough" either.

The pre and post cat O2 sensors are interchangeable, so a free test would be to swap them and see what happens to your trims if your IAT is good. Just make sure to connect them up right so the pre-cat sensor is the new pre-cat sensor. The wiring can reach either one, so it is possible to connect them backwards.
 

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Discussion Starter #11 (Edited)
Any connection like that without preload is going to have some resistance. I would've soldered and heatshrunk, but you can give it a sanity check by reading the intake air temperature when the engine is cold and immediately just cranked. This should be the exact same as your coolant temp which should be the exact same as ambient unless your wiring causes a bad reading. It would have to be really bad to cause 15% error though since it is proportional to absolute temperature. So it would have to read -10°F to think you have 15% more air density. Still, easy to check. This also will only be the case if you messed with the wiring for the supply or reference temp.

My Elise doesn't idle rough, for what that's worth. It isn't my BMW I6, but I wouldn't call it "rough" either.

The pre and post cat O2 sensors are interchangeable, so a free test would be to swap them and see what happens to your trims if your IAT is good. Just make sure to connect them up right so the pre-cat sensor is the new pre-cat sensor. The wiring can reach either one, so it is possible to connect them backwards.
i’ll do that today. and i have a wireless obd2 reader with an app to my phone and i believe i can check the lb/m and temp. from a log i did the intake temp seems to stay 60-80°F. and the lb/min is heavily dependent on rpm and speed, but sits around 1 or less until over 1200 rpm. and goes up to 5.4 or so when driving normally. it changes so quickly it’s hard to claim an average. (from what i can tell).
and the o2 sensor voltage jumps between .120 and .750 going even as low as .020
(measured with an amazon obd2 reader and dashcmd IOS app) and idk if it’s because the app isn’t fully supportive of my particular wireless obd2 unit or something but i’ve never gotten an ambient temp reading from that app. just says “N/A”.

(also i appreciate your help a lot, thanks for staying responsive)
 

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i’ll do that today. and i have a wireless obd2 reader with an app to my phone and i believe i can check the lb/m and temp. from a log i did the intake temp seems to stay 60-80°F. and the lb/min is heavily dependent on rpm and speed, but sits around 1 or less until over 1200 rpm. and goes up to 5.4 or so when driving normally. it changes so quickly it’s hard to claim an average. (from what i can tell).
and the o2 sensor voltage jumps between .120 and .750 going even as low as .020
(measured with an amazon obd2 reader and dashcmd IOS app) and idk if it’s because the app isn’t fully supportive of my particular wireless obd2 unit or something but i’ve never gotten an ambient temp reading from that app. just says “N/A”.

(also i appreciate your help a lot, thanks for staying responsive)
Not a problem. I used to do diagnostics for this kind of stuff. I only did that for a few years so I am far from an expert in it, but I like the practice. It helps me figure out what assumptions I made that shouldn't have, things I missed, stuff like that.

Mass flow is load, so yeah that'll never sit still unless your rpm and throttle input are still. I don't think there is an ambient temp sensor (just pressure), so you'll have to reference an actual thermometer for that. But really, guessing the ballpark is close enough to confirm your MAF sensor IAT reading isn't nuts. I'd say to shelf the idea of a bad MAF or wiring at this point and come back to it if the O2 sensors check out.

I always worked with wideband sensors, so I had to google typical voltages for narrow band. For the voltage to drop to 0.020 is not normal. Did both pre- and post- cat sensors drop that low or just the first one? The ECU only uses the pre-cat sensor for fuel control and the post-cat sensor to see if the cat is doing its job. They should never read the exact same (because of the cat) but they should be in the same ballpark. If the pre-cat was that low and the post-cat was closer to 0.5v, then that strongly puts a finger at the pre-cat sensor.
 

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Discussion Starter #13 (Edited)
Not a problem. I used to do diagnostics for this kind of stuff. I only did that for a few years so I am far from an expert in it, but I like the practice. It helps me figure out what assumptions I made that shouldn't have, things I missed, stuff like that.

Mass flow is load, so yeah that'll never sit still unless your rpm and throttle input are still. I don't think there is an ambient temp sensor (just pressure), so you'll have to reference an actual thermometer for that. But really, guessing the ballpark is close enough to confirm your MAF sensor IAT reading isn't nuts. I'd say to shelf the idea of a bad MAF or wiring at this point and come back to it if the O2 sensors check out.

I always worked with wideband sensors, so I had to google typical voltages for narrow band. For the voltage to drop to 0.020 is not normal. Did both pre- and post- cat sensors drop that low or just the first one? The ECU only uses the pre-cat sensor for fuel control and the post-cat sensor to see if the cat is doing its job. They should never read the exact same (because of the cat) but they should be in the same ballpark. If the pre-cat was that low and the post-cat was closer to 0.5v, then that strongly puts a finger at the pre-cat sensor.
it was the post cat o2 sensor. post cat seems to stay at 0.090 very often, if not consistently, and the pre cat seems to be 0.745-0.750 pretty much most of the time

assuming that post cat is “bank 1 sensor 2” because that’s the one that’s staying so low

edit:

small update: i swapped the o2 sensor locations and wired them up and started the car and cleared the codes, there were no pending codes after the evap and o2 sensors were claimed to be “ready” but i didn’t have my wifi obd2 scanner on me i just had a hardline one. so i’m gonna go back tomorrow or monday and let the car run to check the voltages & graph readings. both sensors seemed very dirty with carbon so we cleaned them up. unfortunately it’s on a lift right now getting painted and body work. so i may not be able to get real results for a little while because i can’t drive it at the moment. hopefully i’ll have it back by next week as he’s painting it tomorrow. and then all that’s left is to piece the grilles back together and clean up the inside. but my inspection is due in 23 days so i’m trying to do as much as i can now.

it idled rough for a few seconds after starting. revs went up and down and up and down and up and down. i almost thought it would stall. but it was its first time running in maybe 3 months since body work started so i thought nothing of it.

next day: so i ran the car for a while and let it warm up all the way to 190°F and the o2 sensors at idle seemed okay. lowest they went was 0.75 on post cat and 0.70 on pre cat.
the highest voltages they both went were
0.835
pre cat averaged 0.513 for about a ten minute idle
post cat averaged 0.481 for about a ten minute idle.
that was after the swap and cleaning^

i also reset the learned values by depressing the throttle 3 times and waiting 20 seconds. the CEL flashed 10 times. and it seemed to run smoother. it didn’t threaten to stall the way i felt it did yesterday. unfortunately i can’t road test it for a few days but that’s where we’re at.
do the readiness sensors become ready from idling? because it said that o2, evap and cat readiness were all not complete. i think i have to drive it though to get them there, right?
 

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There's no preset mileage, just a variety of loads and durations that need to all be checked off. This is usually achieved in 50-200 miles, the more varied the driving the sooner. That being said, you should still be able to get a CEL if you have a fault for two consecutive trips, whether or not readiness is set. So do a varied drive, kill it, repeat a few times. No CEL then should mean you're (hopefully) out of the woods. That would fantastic if they were just sooted up. Fingers crossed!
 

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There's no preset mileage, just a variety of loads and durations that need to all be checked off. This is usually achieved in 50-200 miles, the more varied the driving the sooner. That being said, you should still be able to get a CEL if you have a fault for two consecutive trips, whether or not readiness is set. So do a varied drive, kill it, repeat a few times. No CEL then should mean you're (hopefully) out of the woods. That would fantastic if they were just sooted up. Fingers crossed!
that’s the hope! i’m gonna hopefully drive it from NH to CT and then CT to NH on this monday for a drive that i’m doing anyways. (assuming the car is ready to be driven) but the plan is to get it back saturday 🙏🏻. also do those idle voltages sound correct or proper for an idle reading? or are idle readings too spontaneous to really know what’s good or not, i don’t know much about voltages and car electronics. purely mechanical stuff is moreso my ballpark.
 

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that’s the hope! i’m gonna hopefully drive it from NH to CT and then CT to NH on this monday for a drive that i’m doing anyways. (assuming the car is ready to be driven) but the plan is to get it back saturday 🙏🏻. also do those idle voltages sound correct or proper for an idle reading? or are idle readings too spontaneous to really know what’s good or not, i don’t know much about voltages and car electronics. purely mechanical stuff is moreso my ballpark.
Yeah, I think 500mV is exactly where it should be, and, yeah, idle can be pretty noisy for these sensors. So far everything looks good to go!
 

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Yeah, I think 500mV is exactly where it should be, and, yeah, idle can be pretty noisy for these sensors. So far everything looks good to go!
it ended up coming back with the p0420 engine light after i reset it. and then i put a vibrant performance o2 elbow on it with the medium size flow restriction add-on and it still came back after another light reset and another ~100 miles. should i bother trying the even smaller restriction piece in the elbow? its popping pretty loud when shifting (which i like) but i also need a sticker lol.
One thing i did notice was that the o2 readings with the vibrant elbow AT IDLE looked incredible. the graph showed the postcat sensor super smooth but not suspiciously smooth. it looked healthy. but as soon as i would drive, they would start to look similar and both jump up and down.
 

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it ended up coming back with the p0420 engine light after i reset it. and then i put a vibrant performance o2 elbow on it with the medium size flow restriction add-on and it still came back after another light reset and another ~100 miles. should i bother trying the even smaller restriction piece in the elbow? its popping pretty loud when shifting (which i like) but i also need a sticker lol.
One thing i did notice was that the o2 readings with the vibrant elbow AT IDLE looked incredible. the graph showed the postcat sensor super smooth but not suspiciously smooth. it looked healthy. but as soon as i would drive, they would start to look similar and both jump up and down.
I wouldn't go down this route though because you should already have a functional cat, and adding anything else could produce a false negative since it is going to alter a good signal. Given that things improved when you swapped sensors, I don't think that can be ignored.

Also remember that the post-cat sensor's only job is to evaluate the cat. It does not give the ECU feedback for fuel corrections, that's only the pre-cat sensor, so the elbow won't do anything about the pops if they are there due to a bad reading from the pre-cat sensor. They will be there, however, if the pre-cat sensor is wonky, and, since the post-cat reading is compared to the pre-cat reading for P0420, a bad pre-cat reading can throw the test off. I don't think there's anything that has ruled out the sensors, so I'd go with new ones at this point.
 

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Discussion Starter #20 (Edited)
I wouldn't go down this route though because you should already have a functional cat, and adding anything else could produce a false negative since it is going to alter a good signal. Given that things improved when you swapped sensors, I don't think that can be ignored.

Also remember that the post-cat sensor's only job is to evaluate the cat. It does not give the ECU feedback for fuel corrections, that's only the pre-cat sensor, so the elbow won't do anything about the pops if they are there due to a bad reading from the pre-cat sensor. They will be there, however, if the pre-cat sensor is wonky, and, since the post-cat reading is compared to the pre-cat reading for P0420, a bad pre-cat reading can throw the test off. I don't think there's anything that has ruled out the sensors, so I'd go with new ones at this point.
sounds good. ill try to get new sensors sometime this week. i ended up leaving the sensors in their swapped positions and it never seemed different from before so idk but hopefully that'll help.
do you have o2 sensor suggestions? I'd like a nice easy oem plug and play solution but it seems a lot of them have to be wired into new connectors. also it would probably be for the best to just swap both o2 sensors?

and i know the post-cat elbow wouldn't do anything about the pops, i like the pops, i wasn't trying to get rid of the pops. i was just trying to get the light to shut off. i've read that o2 elbows dont work on 06's specifically though which is just unfortunate. I haven't received p0171 in a while though.

and i’ve noticed a ton of sparkles all over the back of my car lately. which could be from having a brand new resonator & piping in place of a muffler but my first thought is catalytic converter residue. it’s been happening for the past two weeks. it washes off very easy but comes back
 
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