P0420 Code

The catalytic converter sits in the exhaust and cleans harmful gases before they exit the tailpipe. The car monitors it with 2 oxygen sensors that sample the exhaust, one ahead of the converter and another behind it.

With a healthy converter, those sensors give noticeably different readings. Kelley Blue Book notes the code appears when the front and rear sensor signals start looking too alike. A rotten egg odor from the exhaust is one sign you might pick up.

According to Kelley Blue Book, a worn-out catalytic converter is the most frequent culprit. These problems can set it too:

• An exhaust manifold that is damaged or leaking
• An exhaust pipe with damage or a leak
• A front or rear oxygen sensor that has gone bad
• Oxygen sensor wiring that is damaged or loose
• Oil finding its way into the catalytic converter
• A fuel injector that leaks, or fuel pressure that runs too high

The converter is the usual suspect, but a proper diagnosis can save you from replacing a $2,280 to $2,480 part when something else is to blame.

1. Get the emissions system diagnosed, so you learn which fault triggered the code and avoid buying parts you do not need.
2. A mechanic can check for damage or leaks in the exhaust manifold and exhaust pipe, since either can cause P0420.
3. A mechanic can inspect the oxygen sensors and their wiring. Replacing the sensors without a full diagnosis may not clear the code, so it’s worth finding the real cause first.
4. When the sensor readings look normal, ask the mechanic to check for misfires and inspect the ignition and fuel system.
5. Replace the catalytic converter only if the diagnosis shows it has failed.

These figures are published national averages, and a nearby shop may estimate higher or lower.

With converter replacement averaging $2,280 to $2,480, getting a diagnosis first may help you avoid an expensive repair you don’t need.

If your car needs a plug-in emissions test, also called an OBD test, a lit check engine light with P0420 stored will cause it to fail.

Erasing the code right before the test rarely helps either. If you have driven less than about 200 miles since clearing codes or changing the battery, the car’s self-checks may still be unfinished, which can prevent it from passing the test. Guidance used by most states permits just 1 not-ready self-check for cars from 2001 onward, and 2 for cars built from 1996 to 2000.

A few states also examine the converter visually. Georgia includes a visual inspection of the catalytic converter, and Vermont looks at it on every vehicle. When the converter is missing or tampered with, the car fails that check and no waiver applies, so the part must be put back on.

Kelley Blue Book cautions that continued driving with a P0420 code can harm the converter further. So it’s worth getting it looked at soon to help avoid more damage to the converter.