What is the most common cause of P0420?

Catalytic converter genuinely worn out — roughly 40% of cases. Most common past a hundred thousand miles, and the default answer if everything else checks out clean.

How do I diagnose P0420?

Start with: Are there any other codes stored alongside this one? If you have a misfire code or a fuel trim code sitting next to it, stop. Fix that one first, clear everything, and drive it. Those cause this code, and about a third of the time the P0420 never comes back.

P0420

Catalyst System Efficiency Below Threshold (Bank 1)

Name: P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
Uploaded: 2026-08-14
Description: the catalytic converter on bank one isn't cleaning the exhaust as well as the computer expects — or the sensor watching it is lying.

Severity Moderate Safe to drive? Short term System Exhaust / emissions Applies to All OBD-II vehicles, 1996 and newer

In plain English: The catalytic converter on bank one isn't cleaning the exhaust as well as the computer expects — or the sensor watching it is lying.

You can keep driving in the short term. It won't strand you. But you will fail an emissions test, and if the cause is a misfire or a rich condition, you're actively cooking the converter while you wait.

Most likely cause: catalytic converter genuinely worn out (40% of cases). Cheapest first test: are there any other codes stored alongside this one. Typical repair runs $15–$2500 in parts.

What P0420 actually means

There are two oxygen sensors on this exhaust path. The upstream sensor sits before the converter and swings up and down constantly as the fuel trim hunts. The downstream sensor sits after it. A healthy converter stores and releases oxygen, which smooths that signal out, so the downstream sensor should sit lazy and nearly flat, usually somewhere around zero point six to zero point seven volts.

The computer runs a catalyst monitor that compares the two. When the downstream signal starts copying the upstream signal's switching, that means the converter has stopped storing oxygen. Cross the calibrated threshold on two consecutive drive cycles, and you get P zero four two zero.

A measurement below spec — not a part failure report.

Symptoms

Check engine light, on steady
Steady, not flashing. A flashing light is a misfire and that's a different, more urgent problem.
Failed emissions or smog test
This is the one that usually forces the repair, because it's an automatic fail in most states.
A mild drop in fuel economy
Usually a couple of miles per gallon, and often blamed on something else entirely.
A sulfur or rotten egg smell from the tailpipe
That's unburned sulfur compounds passing straight through a converter that isn't reacting anymore.
A rattle from under the floor on cold start
That is the ceramic honeycomb inside the converter broken loose. If you hear it, the converter is mechanically finished.

Most likely causes, ranked

Catalytic converter genuinely worn out 40%

Most common past a hundred thousand miles, and the default answer if everything else checks out clean.

Failed or lazy downstream oxygen sensor 22%

A slow sensor reports a signal that looks like a dead converter. This is the cheap one, and it's why you test before you buy.

Exhaust leak ahead of or near the converter 15%

A leak pulls outside air into the stream and skews both sensors. Fix the leak first or you'll chase this forever.

Engine running rich or lean 12%

Bad fuel trim overloads the converter. If you also have a fuel trim code, that one gets fixed first, always.

Misfire or oil and coolant contamination 7%

Raw fuel or burnt coolant poisons the catalyst material permanently. Fix the source or you'll kill the new converter too.

Aftermarket converter that doesn't meet spec 4%

Cheap universal converters frequently set this code within months, even when brand new.

Downstream O2 sensor — healthy converter vs failed converter

Put your scan tool on live data and watch two sensors at once, with the engine fully warm and holding steady around two thousand RPM.

Flat downstream = converter is fine. Mirrored = converter has failed.

This is a healthy converter. The upstream sensor is switching hard between about zero point one and zero point nine volts, which is normal. The downstream trace stays lazy and nearly flat.

This is a failed converter. The downstream sensor has started mirroring the upstream sensor almost exactly. The converter is no longer buffering oxygen at all.

If your downstream trace is flat, your converter is fine and you're looking at a sensor or a leak. If it mirrors, the converter is the problem.

How to diagnose it, in order

Are there any other codes stored alongside this one?

If you have a misfire code or a fuel trim code sitting next to it, stop. Fix that one first, clear everything, and drive it. Those cause this code, and about a third of the time the P0420 never comes back.

Yes → Fix those codes first, clear, and re-drive

No → go to step 2
Any exhaust leak between the engine and the converter?

Listen at cold idle for a ticking that fades as it warms, and check the manifold, the flex pipe and every gasket. A leak upstream feeds false air to the sensors and produces this exact code with a perfectly good converter.

Yes → Repair the leak, clear, re-drive before condemning anything

No → go to step 3
Are long term fuel trims within plus or minus ten percent?

Check them at idle and at cruise. If they're outside ten percent in either direction, the engine is feeding the converter a mixture it can't process, and that's the actual fault.

Yes → Diagnose the fuel trim fault first

No → go to step 4
Is the downstream O2 sensor responding at normal speed?

Force it rich and lean by snapping the throttle and watch how fast the downstream sensor reacts. A lazy sensor that takes more than about a second to move is the fault itself, not a witness to it.

Yes → go to step 5

No → Replace the downstream O2 sensor
Does the downstream trace mirror the upstream switching?

This is the decisive test, and it's the graph you just saw. Warm engine, steady two thousand RPM, both sensors on screen. Flat downstream means the converter is still working.

Yes → Converter has failed — replace it

No → Converter is fine — re-examine sensor and leaks

Work it in this order and you won't buy a converter you didn't need.

What you need to do this yourself

A scan tool that reads live data — Every diagnosis on this site depends on watching sensor values in real time. A code-reader that only shows the code number cannot do it.
Oxygen sensor socket — A slotted socket that clears the wiring pigtail. A normal socket will not fit.
Digital multimeter — For checking sensor signals and grounds when live data alone isn't conclusive.

Some links may be affiliate links. They cost you nothing extra and do not change which tools are recommended — the diagnostic order on this site is the same either way.

What it costs to fix

Repair	Parts	Labour
Exhaust leak repair (gasket or flex pipe) Cheapest possible outcome, and genuinely common.	$15–$90	1.0 h
Downstream oxygen sensor Usually one bolt and a connector, if it isn't seized.	$40–$140	0.5 h
Catalytic converter, aftermarket direct fit Buy one that is certified for your state, or you'll be back here.	$200–$700	1.5 h
Catalytic converter, OEM Required on some newer vehicles for the monitor to ever pass.	$900–$2500	1.5 h

So the spread on this one code runs from about thirty dollars to well over two thousand. That is exactly why you test in the order we just went through instead of starting with the converter.