Q: Check Engine Light is on - trouble code P0420

As you may know, trouble code P0420 relates to the inefficiency of the catalyst system, which detects the presence of oxygen in the exhaust gases. This can be caused by failing O2 sensors that need to be replaced. This can also happen when the fuel supply is incorrect, supplying too much or too little fuel to the motor which results in the O2 sensors reading too much or too little fuel present in the exhaust gases.

This may happen when the fuel injectors are leaking or working intermittently due to incorrect pulses, an intake air leak, incorrect ignition timing, or even bad spark plugs. I would recommend having an expert, such as one from YourMechanic, come to your location to determine why exactly the Check Engine Light is on and why it is throwing this code.