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P0420 Code - Step-by-Step Diagnostic Procedure

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I have a 2011 1.4L Eco MT, and used to have a P0420 code. Despite many threads saying otherwise, P0420 does not always indicate a bad catalytic converter. I wanted to discuss the diagnostic process I used and give hope to any Cruze owner with a recurring P0420 code. To diagnose, I used a cheap amazon smoke machine (~$180) and the Torque app on my phone with a bluetooth OBD-II reader to read O2 sensor data. To make a long story short, it was a vacuum leak.

1. First and foremost, I made sure the engine was running correctly, by ensuring there were no misfires and that the injectors and spark plugs were firing as they should. I actually changed the spark plugs and coil pack, but the code still returned.

2. I used a smoke machine to pump smoke directly into the exhaust pipe and looked for any exhaust leaks (especially between the two O2 sensors and around the catalytic converter). I didn't have any exhaust leaks, but they can easily trigger a p0420 code by changing O2 sensor readings.

3. I used a smoke machine to pump smoke directly into the air intake to look for any intake leaks. Air intake leaks can also cause a P0420 code by letting excess air (running lean) into the exhaust and changing O2 sensor readings. In my case, there were 3 intake leaks: One where the intercooler pipe mates with the throttle body (loose hose clamp) and two where the intercooler pipes attach to the intercooler. After fixing these air intake leaks, my p0420 code has not returned.

4. If you have completed steps 1-3, and you are running good with no intake or exhaust leaks, then you should remove the catalytic converter and visually inspect the inside honeycomb material for damage, blockage, or severe discoloration. If the catalytic converter looks bad, then you have your culprit. If the catalytic converter looks fine, then you need to reinstall the converter and use a scan tool to look at the O2 sensor readings.

5. Check your O2 Sensor readings. After the car is fully warm, sensor 2 should remain relatively flat around 0.4-0.9V while sensor 1 should switch constantly between 0.1V and 0.9V. Check to make sure this holds true at various engine speeds from idle to 3000RPM. If you see sensor 2 switching voltages like sensor 1, then it is likely your catalytic converter has truly failed.

6. If you have finished the diagnosis, and confirmed you have a bad catalytic converter, you must ask yourself why the converter failed (or it will happen again!). Catalytic converters rarely fail on their own, and usually fail due to engine oil/coolant consumption, PCV problems, excessive misfiring, etc. Essentially, a bad catalytic converter is usually the result of another engine problem sending contaminants into your exhaust system. So before replacing your cat, fix your engine so you won't ever have this problem again.

Hopefully this step-by-step process helps you avoid an expensive repair.
- A GM Engineer
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· Premium Member
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Despite many threads saying otherwise, P0420 does not always indicate a bad catalytic converter.
Totally agree. I have a worn cat at 200K mi. due to previous engine issues. To further clarify, from my own experience with p0420, the "catalyst efficiency" can be made to increase and exceed the threshold by which the p0420 code is set by correcting the engine issues. When monitoring my engine using Torque Pro ($5.00) and an OBD2 bluetooth adaptor ($35.00), I can track the pid "catalyst efficiency" in the "test results" tab. When my engine performance or driving habits contribute to a normal above threshold cat, the listed efficiency range is .35 to 4. My cat is usually .465 to .6, or above the .35 threshold, ie no p0420 code.
When the 420 code and cel is set, the listed efficiency range changes to .5 - 4 and my cat's efficiency has typically dropped to .3, or below the original .35 threshold. My guess for the increased threshold is to force a repair so that the numbers don't bounce back and forth without a resolution to the problem.
The code pops on from my engine's performance during excessive idling, sitting in traffic for hours, and once when I taught my daughter how to park for hours in slow motion;) The code happens a couple times a year.
Although my cat's .3 will increase to .465 just by resuming normal around town or highway speed driving over the course of several drive cycles, it will not exceed the .5 threshold necessary to keep the cel off. So I erase the code and wait for several drive cycles to once again have a "ready" catalyst on my emissions ready tab because the range returns to .35 - 4. And my cat number gradually increases to .6 until the next occurrence.
It is a game until I successfully repair the engine issue. The car runs great and the numbers look normal except under those seldom "extreme" conditions.
Too many words but my point is also, don't buy a new catalytic converter to turn off the p0420 cel without fixing your engine or discovering the cause first. New year's resolution, I gotta getta a smoke machine...
cruzetalk rocks
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