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I've got a 2015 Chevrolet Cruze that recently had the catalytic converter replaced. Everything has been fine with the car, but the CEL keeps coming back on for the catalytic (P0420) and the shop just called to say that the catalytic is running within the correct range and so are the 02 sensors. They think it's an electrical issue that is causing the CEL to come on - could that be the case? If it is a problem with the electrical, what could it be?
 

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I've got a 2015 Chevrolet Cruze that recently had the catalytic converter replaced. Everything has been fine with the car, but the CEL keeps coming back on for the catalytic (P0420) and the shop just called to say that the catalytic is running within the correct range and so are the 02 sensors. They think it's an electrical issue that is causing the CEL to come on - could that be the case? If it is a problem with the electrical, what could it be?
Its sometimes difficult to understand what shops mean or are trying to say, some of them don't know, some of them are dumbing it down so much for the customer that's there's no useful info in their statements and sometimes the service advisor doesn't know what the tech is saying and they just wing it when talking to customer.

Lets assume the best and they mean what they say and know what they are talking about There are many ways an electrical fault can trigger a P0420, however unless it's just a fault in the ECM these electrical faults would be discernible as out of spec readings in the techs viewing of the data.

The statement " everything seems ok, must be electrical" is a common catch-all utterance from techs who can't locate a mechanical concern. Doesn't have any real meaning, it's the same as saying "Ummmm....." When asked what their diagnosis is.

Aftermarket catalytic converters can cause P0420 with no other electrical, fuel or mechanical faults present.
 

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A code P0420 may mean that one or more of the following has happened:
Leaded fuel was used where unleaded was called for (unlikely)
A damaged or failed oxygen / O2 sensor
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
Failed or underperforming catalytic converter (likely) Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination


Read more at: https://www.obd-codes.com/p0420

So as Maven has politely stated, if the diagnosis is correct, you need to look at the original failure, what I mean is, what contaminated the original cat? I would suspect oil contamination myself.

What, if any, were your earlier issues with the vehicle?
 

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Quick check before tracing the original failure as Blasirl recommends - verify the O2 sensors are plugged in correctly to their power supplies. If I remember correctly you have to remove these sensors to replace the Cruze's catalytic converter.
 

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Get yourself an obd2 bluetooth dongle for your car. And the torque pro app for your phone and monitor the o2 sensors.

The front one should bounce around and the back one should stay steady.

If you have a harborfreight in your area or someone that sells infra red temp sensors. Check the temps in front and back of cat. Do a google for interpreting the results. I can't remember if the back is supposed to be hotter or cooler then the front.
 
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