As an experienced service advisor, I saw the P0420 “check engine” light code hundreds of times. This fault code indicates that your car’s catalyst system isn’t performing as efficiently as it should. A vehicle catalyst system, mainly the catalytic converter, reduces harmful emissions. It does so by converting (hence the name) toxic gases the engine discards (like carbon monoxide, hydrocarbons, and nitrogen oxides) into less harmful substances before releasing them into the environment via the exhaust. Let’s break down common P0420 causes, potential fixes, and estimated costs.

Keep in mind that every shop will have its own hourly labor rate and guide times. You can take my estimated labor times and multiply them by your repair facility’s rate.

Also of note: The catalyst system will identify which bank has an issue (if your car has more than one, of course). This helps narrow down which set of sensors, exhaust piping, and catalytic converter is registering a problem. It’s possible for both banks to have problems at the same time, too.

Exhaust leaks

Cause: Leaks in the exhaust system, especially near the catalytic converter or oxygen sensors, can cause incorrect readings and trigger the P0420 code.​

Fix: Repair or replace the damaged section of the exhaust system.​

Estimated cost: Repairing an exhaust leak can take one to three hours. Parts can run between $150 and $500 or more, depending on the extent of the damage.

Faulty oxygen sensors can trigger P0420

Cause: Oxygen sensors monitor the level of oxygen in the exhaust gases. A malfunctioning sensor can send incorrect data to the engine control module, leading to the P0420 code.​

Fix: Replace the faulty oxygen sensor(s).

Estimated cost: Oxygen sensor replacement usually takes about an hour each, unless the old one’s stuck on. Parts can range depending on whether the sensor has advanced features, like an Air Fuel Ratio (AFR) sensor. In general, though, expect to pay a couple hundred bucks for the component.

Engine misfires or running rich/lean

Cause: Engine issues that lead to unburned fuel entering the exhaust can overheat and damage the catalytic converter, resulting in the P0420 code.

Fix: Diagnose and repair the underlying engine issue, which may involve replacing spark plugs, fuel injectors, or addressing vacuum leaks.​

Estimated cost: Repair times and costs vary widely based on the specific issue. Diagnosing the problem may take 1 to 2 hours, with labor costs between $75 and $150 per hour. Parts costs depend on the components needing replacement. Spark plugs and vacuum hoses are relatively inexpensive, think $20 to $60 each. Fuel injectors, though, are pricier – expect to pay several hundred bucks apiece.

All underlying issues can lead to the most common P0420 fix: Catalytic converter replacement

Cause: Over time, catalytic converters can degrade due to age, contamination from engine oil or coolant, or damage from misfires. Basically, all of the above issues we’ve already discussed can still lead to converter failure. If it can’t effectively reduce harmful emissions, it triggers the P0420 code.​

Fix: Replace the catalytic converter.​ Note that it’s really important to address the source of contamination (like engine leaks) or you’ll risk ruining the new one.

Estimated cost: Replacing a catalytic converter typically takes about 1 to 2 hours. Labor costs range from $100 to $300, while parts can be between $500 and $2,000, depending on the vehicle make and model. As such, converter replacement is the priciest issue to resolve the P0420 fault code.

Please remember that these are average estimates. Actual costs can vary based on your location, vehicle make and model, and the specific repair shop’s rates. It’s always a good idea to get multiple quotes and ensure the shop uses quality parts.​

Addressing the P0420 code promptly can prevent further damage to your vehicle and ensure it runs efficiently. I’ve seen failed converters blow engines, so again, it’s important to address promptly if you notice any performance issues. If you’re unsure about diagnosing or repairing the code yourself, consult a trusted mechanic.