what is engine code p0420

Asked by May 25, 2013 at 02:12 PM about the 1994 Honda Civic

Question type: Maintenance & Repair

5 Answers

Catalytic Convertor and/or downstream O2 sensor issue. Some forums and other websites say the O2 sensor is not involved, others do, but what they all agree on is the cat. "Below Efficiency Threshold"

2 of 2 people found this helpful.
3,410

Correct. The rear oxygen sensor is telling the computer that the catalytic converter is not working efficiently enough. So really it could be either part. Thise problem is best diagnosed by someone with a professional level scan tool as it requires reading the waveform and switching rate of the oxygen sensor. Of course if you dont mind spending money, you could just replace the rear oxygen sensor itself and hope thats it.

4 of 4 people found this helpful.
155

P0420 is simply "cat efficiency below threshold" Borrow a scan tool that has the appropriate obd1 connector for the car, and look at the PID data. You should see the voltage fluctuate from like .1v to.9v on a working sensor. IF YOU ARE RUNNING AN AFTERMARKET CATALYTIC CONVERTER OR TEST PIPE, YOU WILL THROW THIS CODE... If that is the case, simply search on a site like passwordjdm.com for a mechanical o2 sim. That will fix that issue.

4 of 4 people found this helpful.
Best Answer
1,555

Catalyst System Efficiency Below Threshold (Bank 1) you can look up more info on http://www.obd-codes.com/p0420 this will give you afew things to look at for possible causes and will give you some ideas of where to look to fix it.

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My first time using Car Gurus! You guys are awesome and I really appreciate the feedback.

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