Question

How Do I Fix the Error Code P0420 on a Chevy?

The DTC P0420 or the 'catalyst system efficiency below threshold' code can occur when your Chevy’s catalyst system is not working as well as it should. Here is a step-by-step method on how you can fix it: 

Step 1- Fix other codes: First, resolve any other codes that may have come up besides the P0420; especially troublesome is the P0300 series of cylinder misfire codes. 

Step 2– Check and resolve exhaust system leaks: Pressurize the exhaust system with compressed air and spray soapy water around suspected areas to detect any leaks. Once the leak is detected, fix it using exhaust tape, epoxy, gasket replacement or replace the part. 

Step 3- Check for cylinder imbalance: Run a cylinder imbalance test to check if one cylinder is running lean and the other cylinders are running rich. For this you could make use of an advanced diagnostic scan tool.  

Step 4- Check your spark plugs: Another cause of high emissions is worn-out or faulty spark plugs. Check it thoroughly and while at it make sure to clean your catalytic converter efficiently. 

Step 5- Replace the Catalytic Converter: If your PCM still registers the code, chances are the problem is with your Chevy’s catalytic converter. If it has genuinely failed, replacement is the only option.