2011 Honda Accord
Mileage
111,100
Service Date
2/16/2026
Services Performed
- Inspected and found a p0420 – catalyst efficiency b1 below threshold set in the ECM as a temporary and permanent code. First thing I checked the fuel trims to verify the vehicle was not running rich causing the catalytic converter to malfunction, the fuel trims were sitting at +7 at idle and +9 with steady throttle driving at 45mph which is ideal and within spec. Next while I was on the text drive I had a partner monitor the rear O2 sensor data while I was steady on the throttle keeping consistent RPM and speed, and found that the rear O2 sensor was rapidly switching from rich to lean back and forth indicating that the catalytic converter is not properly storing oxygen, with a healthy catalytic converter the rear O2 sensor should stay steady on rich or lean and should not rapidly switch. At this time I would recommend replacement of the catalytic converter and the upstream A/F sensor (Honda strongly recommends replacement of the A/F sensor when replacing the catalytic converter by TSB 14-002 due to the faulty catalyst possibly damaging the internal cap in the A/F sensor) While I had the vehicle in the bay I decided to check the spark plugs and found that the spark plugs are starting to wear and need to be replaced. They are not the cause or symptom of the P0420, but will soon cause a misfire. At this time I would recommend replacement of all 4 spark plugs.
- Removed cat converter. Installed new one using 2 new gasket. Bolted to spec and then tested. Cleared codes and test drove. No longer have any active dtc.
- Removed and replaced spark plugs with new.
Cathy W. gave our service a 5 star review on 2/21/2026
