P0420 Code: Is Your Catalytic Converter Failing?

P0420 means catalyst efficiency below threshold. Learn what causes it, why it's not always the converter, and how it's diagnosed.

RCJ Mobile Mechanics

10/12/20261 min read

Close-up of blue exhaust pipes on a car
Close-up of blue exhaust pipes on a car

P0420 is a code that worries a lot of drivers, because catalytic converters are expensive. But a P0420 code doesn't automatically mean you need a new one.

What the code means

P0420 stands for "Catalyst System Efficiency Below Threshold, Bank 1." Your car has an oxygen sensor before the catalytic converter and another one after it. The engine computer compares the two. If the readings after the converter look too similar to the readings before it, the computer decides the converter isn't cleaning the exhaust well enough.

What causes P0420

  • A worn-out or damaged catalytic converter

  • A faulty downstream (rear) oxygen sensor giving a wrong reading

  • Exhaust leaks near the oxygen sensors

  • Misfires that sent unburned fuel into the converter

  • The engine burning oil, which contaminates the converter

  • A rich fuel mixture from leaking injectors or bad sensors

Why the converter fails

Catalytic converters usually fail because of another problem upstream. Long-term misfires, oil burning or coolant leaks into the combustion chamber can melt or clog the inside of the converter. Replacing the converter without fixing the cause often means the new one fails too.

Symptoms

  • Check engine light

  • Rotten egg (sulphur) smell from the exhaust

  • Reduced power, especially if the converter is clogged

  • Rattling from underneath, from a broken internal core

Sometimes there are no symptoms other than the light.

How it's diagnosed

A good diagnosis checks for exhaust leaks, tests both oxygen sensors, looks for misfire history and checks fuel trims. Only after ruling out those causes should the converter itself be blamed.

Can you keep driving?

Usually yes, in the short term, if the car is running normally. But don't ignore it. A clogged converter can restrict the exhaust and cause serious power loss and engine heat.

Need help with this? RCJ Mobile Mechanics comes to you in Portmore, Spanish Town, Kingston, St. Andrew and across St. Catherine. Call or WhatsApp (658) 207-6565 or book a mechanic online.