Questions and Answers
Honda Accord DTC P0420
Q. Dear Vincent Ciulla, I just found out my 1998 Honda Accord LX sedan 4 cylinder 2.3 Liter VTEC with an automatic transmission has OBD II code P0420 for "catalyst system efficiency below threshold (bank 1)". This started when the car had about 40,000 miles.
The car still runs great and I haven't noticed any change in gas mileage or performance. I am not having any other problems with the car. I change the oil, filters, and spark plugs regularly. Most of the miles on this car are highway miles. The first time the check engine light came on I had just driven for 4 hours on the highway to Dallas, TX.
Although I used to work on cars frequently, I have not worked on them nearly as often as I did 10 years ago. When I think of banks, I think of cylinder banks. I doubt that is what they are talking about. What do they mean by bank one?
I have 2 of these cars. I took the O2 sensor out of the exhaust manifold of the car that does not have any problems and switched it with the car that is having a check engine light come on. It did not change anything on either car even after the computer was reset.
I have had an emissions test run, but all of the emissions were well below the required levels for Harris County, TX ( a fairly strict county).
A technician at a local Honda dealer told me it is probably an exhaust leak, but I cannot find an exhaust leak anywhere from the engine back to the tailpipe. I have gotten underneath the car with it running and still cannot detect a leak. If the catalytic converter is bad, wouldn't the car fail the exhaust emissions test?
Next year any car in Harris county that has any codes show up is automatically failed regardless of the exhaust emissions test. What would you check next to troubleshoot this code? I want to diagnose this problem correctly before I buy any parts if possible.
Sincerely,
JohnA. DTC P0420 is "Catalytic System Efficiency Below Threshold". What this means, in a nut shell, is the Three Way Catalytic Converter (TWC) is not working properly. To confirm this, you would need a scan tool. The tech at the Toyota dealership made a good guess. A leaking exhaust could cause the P0420 but the leak would have to be between the TWC and the exhaust manifold.
Bank one and bank two still refer to cylinder head banks, but since this is a four cylinder it only has one bank.
Additional Information provided courtesy of ALLDATA
