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P0421: Warm up catalyst efficiency below threshold (bank 1)

The small warm up catalytic converter on bank 1 is no longer cleaning the exhaust well. It may be worn out, or an oxygen sensor may be misreading.

Code P0421 at a glance
CodeP0421
SystemEngine and transmission
GroupP04xx, Emissions control
TypeGeneric: it means the same on every make
Can you keep driving?With care

What P0421 means

Some cars have a warm up catalyst close to the exhaust manifold so it starts working quickly from cold. The ECU compares the oxygen sensors before and after it and has found that it is no longer storing and converting gases well. Bank 1 is the side of the engine with cylinder 1; inline engines only have bank 1.

Most common causes

  • Exhaust leak near the oxygen sensors (gasket or pipe)
  • Slow or faulty downstream oxygen sensor (sensor 2)
  • Corroded oxygen sensor connector
  • Worn out or damaged catalytic converter
  • Misfires or oil burning that have damaged the converter

Symptoms

  • Engine warning light on
  • Failing an emissions test
  • Rotten egg smell from the exhaust
  • Sometimes loss of power if the converter is blocked

Can you drive with P0421?

With care You can drive, but if there are misfires or the converter becomes blocked, it can get worse. Take it to a garage soon and before the next inspection.

How to check it

With the engine warm, watch the oxygen sensor voltages on your reader: the upstream one (sensor 1) should switch quickly and the downstream one (sensor 2) should stay fairly steady. If both switch the same way, the converter is worn.

How to read and clear P0421 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0421 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

P0421 is a generic code, so it means the same on any make, for example on Audi, Fiat, Mazda, Opel / Vauxhall, Peugeot, Toyota. See what Final OBD does on your car in makes and models.

What does code P0421 mean?

Some cars have a warm up catalyst close to the exhaust manifold so it starts working quickly from cold. The ECU compares the oxygen sensors before and after it and has found that it is no longer storing and converting gases well. Bank 1 is the side of the engine with cylinder 1; inline engines only have bank 1.

Can you drive with P0421?

You can drive, but if there are misfires or the converter becomes blocked, it can get worse. Take it to a garage soon and before the next inspection.

How do you clear P0421?

With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0421 comes back.