Engine Fault Code P0420 – Symptoms, Causes, Fix

When you get a P0420 code it will read “Catalyst system efficiency below threshold (bank 1)”. This points to an issue with your catalytic converter. However, there are a number of issues that can cause this beyond just the catalytic converter.

P0420 engine codes can be caused by a bad catalytic converter, exhaust leaks, or bad oxygen sensors. In this guide we’re going to discuss the signs and symptoms along with how to diagnose what is causing the P0420 code.

P0420 Symptoms

• Check engine light (CEL, MIL, etc)
• Raw exhaust smell
• Poor fuel economy
• Misfires
• AFR lean or rich

Scanning the engines computer via the OBDII port and seeing the code pop up will be the most obvious give-away. Exhaust leaks or poorly functioning catalytic converters may result in a raw exhaust smell. This is similar to rotten egg or sulfur smells. In some cases you may notice poor fuel economy.

Misfires, air-fuel ratios too lean or rich, power loss, and misc. symptoms may also occur. However, these are typically symptoms of another underlying issue that’s potentially causing DTC P0420, such as issues with the O2 sensors.

Causes of P0420 Engine Codes

• Worn/failing catalytic converters
• Lack of cats
• Exhaust leaks
• Failing O2 sensors

Catalytic Converter

Worn, failing, or missing catalytic converters are usually the most common cause of DTC P0420. It’s not uncommon for them to become less effective with age, especially after 10+ years and 120,000+ miles. Catalytic converter theft is, unfortunately, a common thing so you could be missing the cats all together.

Exhaust Leaks

Exhaust leaks are another common reason to get the fault code. The exhaust system can rust and develop holes. Exhaust manifold gaskets or manifold cracks also occur sometimes. This creates an air leak, allowing exhaust gases to escape which can trigger the O2 sensors to throw a check engine light, thinking the issue is with the catalytic converter.

O2 Sensors

The last thing to look at is the oxygen sensors. The cats may be in good working order, but a bad O2 sensor can make the car believe something is wrong with the cats. Misfires and rich AFR’s can be underlying issues that actually result in catalytic converter failure.

Diagnosing P0420 Codes

When diagnosing any engine problem it’s important to get the full picture. If there are other diagnostic trouble codes present they may relate to this one. Here are the steps to take to diagnose what part is causing your code:

1. Ensure P0420 is the only engine code present
2. Clear code and drive for about 15 minutes. Check to see if the code comes back. If not, continue this process a few times to confirm. If DTC P0420 comes back then there’s an actual issue at hand
3. Physically inspect exhaust system for any damage, holes, rust, or other potential leaks. Don’t forget the exhaust manifold as that’s a common area for leaks, too.
4. Check O2 sensor voltage reading. Standard readings should be around 0.4 to 0.5 volts on most engines. If it’s wildly off or bouncing around then it’s likely an issue with the catalytic converter

Sometimes engines have minor hiccups and fault codes randomly appear. In these cases simply clearing and deleting the code may fix the problem. Otherwise, it’s time to track down what’s actually causing it. The above steps are pretty effective for exhaust leaks and cat failure. However, if you’re getting other engine codes then it’s going to take a bit more ingenuity and problem solving.

Replacement Options

What are the repair costs for engine fault code P0420? Below is a breakdown of repair costs for some common fixes:

• Catalytic converter: $200-2,000+
• O2 Sensors: $50-300
• Exhaust leak (weld): $0-200
• Exhaust leak (new exhaust): $200-500+

Catalytic converters are complex emissions equipment that use some expensive materials. Unfortunately, that means cat replacement is not a cheap job. The parts alone can run $200 if you source used converters. Going with new OEM parts can exceed $2,000 on some engines. This is especially true if multiple catalytic converters require replacement.

An O2 sensor can run as low as $50 new on some cars. However, $75-150 is probably a more realistic number for most models. They’re generally easy to replace but labor can be another hour or two.

Lastly, P0420 exhaust leak repairs can vary a lot depending on the fix you decide on. Welding can be free (or very close to it) if you have the tools and experience. At a repair shop it will probably be about $100-200 to weld the holes. Opting for a new exhaust can quickly add up to $500+ if you’re going with new parts.

Frequently Asked Questions