* P0135: This code refers to a Heated Oxygen Sensor (HO2S) Heater Circuit Malfunction (Bank 1, Sensor 1). Bank 1 refers to the side of the engine containing cylinder #1. Sensor 1 is the upstream sensor (before the catalytic converter). This means the heater element within the oxygen sensor on that bank is not working correctly. The oxygen sensor needs this heater to function properly, especially when the engine is cold. A faulty heater can lead to inaccurate readings and a rich or lean condition.
* P0420: This code means Catalyst System Efficiency Below Threshold (Bank 1). This indicates a problem with the catalytic converter on Bank 1. The catalytic converter isn't efficiently converting harmful emissions. This could be caused by a failing catalytic converter itself, a problem with the upstream O2 sensor (which is related to the P0135 code), or potentially a problem with fuel delivery or ignition causing too much unburnt fuel to reach the converter.
The relationship between the two codes: The P0135 code (bad O2 sensor heater) is likely *contributing* to the P0420 code (bad catalytic converter). An inaccurate or slow-to-respond O2 sensor due to a faulty heater can cause the catalytic converter to work harder and eventually fail. However, a failed catalytic converter can *also* cause a P0135 code in some cases (a feedback loop).
What to do: You should have a qualified mechanic diagnose and repair these codes. Simply replacing the oxygen sensor might solve both problems, but it's crucial to have the system properly checked to ensure the catalytic converter isn't already damaged and needs replacement (which is an expensive repair). A scan tool can help pinpoint the exact issue, and a mechanic can verify the condition of both the sensor and the catalytic converter.
