P0420 Catalyst System Efficiency Below Threshold

P0420 is one of the most common trouble codes, and one that shouldn’t be ignored. This diagnostic trouble code is related to the catalytic converter in your vehicle. The catalytic converter is a part of your exhaust system and it’s meant to reduce emissions.

On many cars, there are more than one catalytic converter. Often a primary, which is attached to the header or right after a header – and then a secondary, which is often smaller and on a mid-pipe. That’s a typical car setup, but every vehicle is different.

P0420 Catalyst System Efficiency Below Threshold trouble code

This code is caused due to an oxygen sensor getting a bad reading from the exhaust. There will be an oxygen sensor before and after the cat. Many times, when an aftermarket performance header is installed and the primary cat is removed – you’ll get a check-engine light. This is because the oxygen sensor after the cat is getting a bad reading.

Basically, the upstream O2 sensor reading will fluctuate and the downstream sensor reading is steady. P0420 is triggered when both readings are similar – obviously signaling that something isn’t right with your exhaust.

Catalytic converters are typically meant to last a lifetime, but if your vehicle hasn’t been maintained properly then it is possible the cat has become clogged.

Let’s get into the symptoms and causes for P0420…


The most common symptom associated with P0420 is a lack of power. Many drivers will experience reduced power when accelerating due to the exhaust being restricted. Other than that, a check-engine light is the only other symptom drivers seem to notice.


  • Failed catalytic converters
  • Leak from either the exhaust manifold, cat, muffler or exhaust pipe
  • Engine coolant temperature sensor not working properly
  • Faulty oxygen sensor
  • Downstream O2 sensor wiring damaged (very common if you go off-roading)
  • High fuel pressure / leaking fuel injector
  • Misfire – usually the cause of catalytic converter failure
  • Engine running lean or rich – improper air-fuel ratio can cause catalytic converters to fail

This is a very serious trouble code and it should be addressed as soon as possible to prevent damage to your engine. If the catalytic converter has failed, you will need to replace it right away.

Diagnosing P0420 and Fixing the Problem

Don’t make the common mistake of assuming its a faulty oxygen sensor! This is a very common mistake made, and you shouldn’t waste money on parts you don’t actually need.

First, scan the vehicle with an OBDII reader and make sure it’s the only code present at the time. If there are other codes, then make sure to address them first, as they might be the cause of P0420.

Next, you’ll want to check for exhaust leaks. Run the engine, and inspect the exhaust manifold, and any connections made before the catalytic converter. Inspect the gaskets and make sure all the connections are tight. Found a leak? Repair it, clear the code, then test drive vehicle to see if code comes back.

Testing the downstream O2 sensor with a multimeter…

If you still have the code, then you’ll need to grab a multimeter and inspect the oxygen sensors before and after the cat. Have the vehicle running while you perform these tests.

You’re going to check the voltage reading of the downstream oxygen sensor to make sure it shows a steady reading of about 0.45V.

However, if that reading is jumping around from 0.1V to 0.9V – the catalytic converter is bad and needs to be replaced.


Catalytic converters are not cheap, so we advise you to inspect thoroughly and test the downstream oxygen sensor with a multimeter before making that leap. Cats run anywhere from $300 to $2500, and often require welding to install. On some vehicles, you can simply buy an OEM header with primary cat installed already, but it’s still not cheap.

If you’ve thoroughly inspected your exhaust system for leaks, fixed all other trouble codes, and tested the oxygen sensors, then it’s time to replace the catalytic converter. Try buying used from a junkyard if possible.

For those who installed an aftermarket exhaust and deleted the cat, you will have to look into finding a de-fouler to trick the system into thinking the cat is there and working properly.