Q: Po420 bank1

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My van runs great when I started I get a knock which gets better as it warms up, my fuel trims seem to be high.ltft. I clear the code but after about 40 miles it comes on again. Thx tom

My car has 125000 miles.
My car has an automatic transmission.

Hello Tom. P0420 is the Catalyst System Efficiency Below Threshold (Bank 1) code. While there are a multitude of components that can cause this code, the general concept is that your catalytic converter is not working properly. If you suspect the catalytic converter is not to blame, then the oxygen sensor after the catalytic converter may not be working correctly. The only option at that point is to replace the sensor. You will also want to visually inspect the exhaust system. Check for any cracks, holes, discoloration, and even tap on the catalytic converter to see if there is a rattle inside. A hole, crack, or dent would restrict the flow of exhaust and give false readings to your sensors. As well as stress the engine, as it will try and compensate for the changes. When tapping on the catalytic converter, you are listening for a rattle or indication that the converter has failed and come apart on the inside. Again, check for any signs of damage, and start by replacing the oxygen sensor if needed. Turn to the catalytic converter next if the issue continues.

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