Exhaust and catalyst

P0420Catalytic converter efficiency below threshold, bank 1

Catalyst System Efficiency Below Threshold Bank 1

Get it diagnosed soon

Generic OBD-II code

Catalytic Converter

What this part does

Uses a catalyst bed to convert exhaust pollutants (CO, HC, NOx) into less harmful gases; efficiency is monitored by comparing upstream/downstream oxygen sensor activity.

What the computer is measuring

There are two oxygen sensors in this part of the exhaust: one ahead of the catalytic converter and one behind it. The front sensor is there to run the fuel mixture. The rear sensor exists almost entirely to grade the converter.

A working converter stores and releases oxygen as it burns off the leftovers in the exhaust. That buffering action smooths the signal out, so the front sensor swings up and down constantly while the rear sensor stays comparatively flat. The computer watches the two traces and compares how much the rear one moves against the front one.

When the rear sensor starts mirroring the front, the converter has stopped buffering. After the computer sees that across enough drive cycles, it sets P0420 and turns the light on. Bank 1 is the side of the engine with cylinder number one on it, which on a four cylinder is simply the only bank there is.

The important thing this test cannot tell you is why the buffering stopped. A dead converter and a dead rear sensor produce a similar looking trace, and so does an exhaust leak pulling fresh air across the sensor. That is the whole reason the checklist below starts with the other codes rather than with a parts order.

What you will notice

  • The check engine light is on steady. It rarely flashes for this one.
  • Usually nothing else. The car drives normally, which is why it is often found at an emissions test.
  • Sometimes a faint sulphur or rotten egg smell from the exhaust.
  • A drop in fuel economy if the fault that damaged the converter is still present.

Likely causes, most common first

Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.

  1. A worn out catalytic converter

    Genuinely the most common answer on a high mileage car, but it is the last thing to conclude, not the first. Converters usually die because something else killed them.

  2. A lazy or failed downstream oxygen sensor

    The rear sensor is the only thing reporting converter performance. A slow one reports a converter that is fine as one that is not.

  3. An exhaust leak ahead of or between the sensors

    Air drawn in through a cracked manifold or a bad flange gasket skews the rear sensor reading and sets this code with a healthy converter.

  4. An untreated misfire or a rich running condition

    Raw fuel burning in the converter overheats and destroys it. If P0300 or a fuel trim code is also stored, fix that first or you will kill the new converter too.

  5. Engine oil or coolant entering the exhaust

    A leaking valve stem seal or head gasket coats the catalyst and poisons it permanently. Look for blue or white smoke and check consumption.

What to check, in order

  1. Read every stored code first, not just this one.

    If a misfire or fuel trim code is present alongside it, that is the job. Replacing the converter without fixing the cause repeats the failure in months.

  2. Look and listen for exhaust leaks from the manifold back to the rear sensor, with the engine cold and then warm.

    A leak found here explains the code on its own. Fix it and clear the code before going further.

  3. Watch both oxygen sensors live while the engine is warm and holding steady around 2000 rpm.

    The front sensor should swing quickly and constantly. The rear should sit fairly flat. A rear sensor swinging in step with the front points at the converter. A rear sensor that barely moves at all, or moves lazily, points at the sensor.

  4. Compare the two sensors on a bank 2 engine, if the vehicle has one.

    A V6 or V8 with a healthy bank 2 and no P0430 gives you a known good reference for what the rear sensor trace should look like.

Where this one turns up most

  • High mileage four cylinder Honda and Toyota engines, where the original converter simply reaches the end of its life.
  • Vehicles that have had a misfire or a rich condition left unrepaired for a long period.

Codes that often show up alongside it

What a DriveDecode adapter can read

A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.

Reading P0420 on your own car

DriveDecode pulls P0420 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.

See how DriveDecode works

Last reviewed 2026-09-05. Definition sourced from the published standard.