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P0420 and rough idle?

4801 Views 6 Replies 5 Participants Last post by  sturgeongeneral
Hi guys I have 2006 Lotus Elise,
Had to take out the gearbox and change a two gears last week. Car put back together and gears working perfectly. Unfortunately I have noticed since getting the car back that the idle is rough (up and down) Under power it seems to be working fine but in traffic sometimes feels like it wants to stall.
Drove car like this for 3 days and then on Friday got the P0420 Catalyst System Efficiency below threshold code. Cleared code this morning and it has not come back yet but the idle is still iffy.
I have on a stage 2 exhaust that I bought the car with and no decat. Do you think I have a faulty catalytic converter? Faulty o2 sensor? Dirty MAF? Damaged exhaust? What are some fixes for fluctuating idle?
Thanks in advance for any advise!
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· Registered
2,012 Posts
is it just a rough idle, or does it misfire when driving as well? & is there a pattern to when it starts to 'run rough' like only during cold starts or also when the engine is warm?

a clogged cat could be the case, but unlikely since it was fine prior to the service you just got (doubt its a bad o2 sensor as those shouldn't cause rough running issues on the elise)

the catalyst code for our cars are sort of strange, you would think it immediately means bad CAT, but based on other threads it tends to be anything but that since the rough idle is prominent

from what your explaining makes me think that its possibly a bad ground/sensor not seated correctly/air leak at the intake/maf

with that said, check to make sure the intake tube clamps between maf to the throttle body is tight/secure (including the maf & maf harness itself).

also check the ground wire from the engine harness, specifically the smaller one right below the oil fill cap that bolts to the head (coincidentally above the transmission & very well could have been unbolted/messed with during that service)

there are a few other pointers ill give based on your response to this reply (for the sake of not writing a novel yet)

· Registered
8 Posts
Discussion Starter · #3 ·
Thanks for your quick response driftwood! It is just a rough idle, not misfiring under power. It happens when the car is hot or cold. Sometimes have to engage clutch while sitting in traffic so as not to stall but seems fine once engine revs are over 2000. I have printed off your suggestions and will take them with me when I return to the garage that did the work for me. Fingers crossed!

· Vendor
6,298 Posts
perhaps they switched the 02 sensor leads during the install... that would potentially exhibit these issues.

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