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P0420 CEL was on, but it went away?

Old 03-13-2019, 11:25 PM
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Default P0420 CEL was on, but it went away?

Hey guys, keeping this short and sweet.

I had a CEL come on 6 days ago with the code P0420. Did quite a bit of digging and decided this was something I needed to bring my car into the shop for since I have no time or space to check into it. Yesterday morning the CEL went away on the way to work and after driving around 80 miles since then it has yet to come back on. Does it coming on and off help diagnose this for anyone?

Let me know if anyone has any ideas; I'm not sure if this is a real problem or not. Thanks!
Old 03-16-2019, 08:31 AM
Join Date: Nov 2017
Location: Isanti, MN
Posts: 828

P0420 Catalyst System Efficiency Below Threshold


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

Possible Solutions

Some suggested steps for troubleshooting and fixing a P0420 error code include:
  • Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
  • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • Inspect the downstream heated oxygen sensor, replace if necessary
  • Replace the catalytic converter
Would be worth getting checked out, hopefully it's something easy. Good luck
Old 03-17-2019, 07:11 PM
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Thanks for the information! Sure is a lot to take in....the light hasn't came back on yet so I'm going to bookmark this thread just in case it does so I can take appropriate action.

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