Questions and Answers

Honda Accord DTC P0420 Q. Hello Vincent, My daughter has a 1998 Honda Accord with an intermittent check engine lamp that comes on. Ran it to Autozone with the diagnostic scanner and the code comes out as P0420 with a definition of "Catalyst efficiency below threshold" (Bank 1). Please decode that for me as they didn't know what it meant. Also what the possible problem could be. Thanks, Pat

A. DTC P0420 - Catalytic System Efficiency Below Threshold indicates a catalytic converter that is not working properly.

The PCM uses readings from the front and rear O2 sensors to monitor the efficiency of the catalytic converter. If the readings get within a certain range, the PCM sets this code and turns the Check Engine Light on.

It could also be set by a cracked exhaust manifold or a leak in the exhaust system.