what is engine code p0420


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

Question type: Maintenance & Repair

5 Answers

Not Active

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"

1 out of 1 people think this is helpful.
Michael Mierzejewski

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.

3 out of 3 people think this is helpful.
John Surovik

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.

2 out of 2 people think this is helpful.
Best Answer
Bradley Shelledy

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.

2 out of 2 people think this is helpful.

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

Your Answer

Add photo

Related Questions

Looking for a Used Civic in your area?
CarGurus has 55,557 nationwide Civic listings starting at $999.

Search Honda Civic Questions

Honda Civic Experts

#1 Takoda Fraser
Takoda Fraser
Reputation 840
#2 Tom Demyan
Tom Demyan
Reputation 650
#3 Jonathan Ford
Jonathan Ford
Reputation 580
View All

Related Models For Sale

Used Honda Accord"
876 Great Deals out of 71,797 listings starting at $1,295
Used Honda Civic Coupe"
270 Great Deals out of 9,884 listings starting at $1,300
Used Toyota Corolla"
1,019 Great Deals out of 69,891 listings starting at $1,795
Used Toyota Camry"
1,100 Great Deals out of 82,621 listings starting at $1,499
Used Nissan Altima"
1,062 Great Deals out of 80,739 listings starting at $950
Used Hyundai Elantra"
643 Great Deals out of 46,711 listings starting at $795
Used Nissan Sentra"
665 Great Deals out of 48,426 listings starting at $1,795
Used Ford Mustang"
609 Great Deals out of 41,268 listings starting at $1,695
Used Volkswagen Jetta"
774 Great Deals out of 31,765 listings starting at $500
Used Toyota Tacoma"
494 Great Deals out of 18,815 listings starting at $2,000
Used Acura TL"
214 Great Deals out of 6,358 listings starting at $900

Used Cars For Sale

2016 Honda Civic For Sale
10,719 listings
2015 Honda Civic For Sale
42 Great Deals out of 25,069 listings starting at $13,499
2014 Honda Civic For Sale
60 Great Deals out of 1,966 listings starting at $11,888
2013 Honda Civic For Sale
155 Great Deals out of 3,538 listings starting at $9,491
2012 Honda Civic For Sale
257 Great Deals out of 5,607 listings starting at $7,885