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P0420 – Catalyst system efficiency below threshold (Bank 1)

What it means

The engine computer compares the oxygen sensor in front of the catalytic converter with the one behind it. A healthy converter stores oxygen, so the rear sensor signal stays calm and steady. If the rear signal swings almost as much as the front one, the computer concludes that the converter is no longer cleaning the exhaust well enough and sets P0420. "Bank 1" is the side of the engine with cylinder 1. An inline engine has only one bank.

Symptoms

• Check engine light on. Often this is the only sign.
• Less often: a rotten-egg smell, a rattle near the converter or a slight loss of power.
• A failed emissions test.

Common causes (most likely first)

1. An aged or contaminated catalytic converter.
2. A slow or faulty rear oxygen sensor, or its wiring.
3. An exhaust leak in front of the converter or between the sensors. Outside air then skews the reading.
4. Earlier misfires or a rich mixture that overheated or fouled the converter.
5. Burning oil or coolant, which coats the converter.
6. Occasionally engine software. Some makers release an update for this.

How urgent is it? Can I drive?

If P0420 is the only code and the engine runs normally, the car is usually fine to drive in the short term. Have it checked within a few weeks, because another fault is often behind it. If the check engine light is flashing, that means misfires. Back off the throttle right away, because unburnt fuel can ruin the converter within minutes.

What to check first

1. Read all stored codes with a scanner that shows live data. Fix misfire, oxygen sensor and mixture codes (P0300, P0171 and so on) first.
2. Watch both oxygen sensors in live data with the engine fully warm and a steady throttle. The front sensor should switch quickly. The rear one should be much calmer. If the rear one switches just like the front one, that points to the converter. If the rear signal never moves at all, the sensor may be faulty.
3. Look for exhaust leaks. On a cold start, puffing at a joint or a sooty streak gives a leak away.
4. Check the freeze frame, which shows the engine speed and temperature when the code was set.

When to see a mechanic

See a mechanic if the code returns after clearing, if other codes are present, if the light flashes or if you have no scanner with live data. A converter is expensive. Don't replace it without a proper diagnosis that has ruled out the sensor and leaks first.

FAQ

Will a new rear oxygen sensor fix it?

Only if testing shows the sensor is bad. If the sensor is fine and the converter is worn, the code will come back with a new sensor too.

Can the converter be removed or the rear sensor coded out?

No. Removing the converter or disabling its monitoring is illegal, and we don't help with it.

Can misfires cause P0420?

Yes. Misfires send unburnt fuel into the converter and overheat it. That's why misfires always have to be fixed first.