P0421 and P0422: warm-up catalyst efficiency below threshold

Many engines have two catalysts: a small one bolted close to the exhaust manifold, and a larger one further down. The close-coupled unit is the warm-up catalyst, and it exists because it reaches operating temperature in seconds rather than minutes.

P0421 and P0422 report that this specific converter is not converting well enough.

Code	Meaning
P0421	Warm-up catalyst efficiency below threshold, bank 1
P0422	Main catalyst efficiency below threshold, bank 1
P0431, P0432	The bank 2 equivalents

The general catalyst discussion applies: P0420 catalyst code explained and P0430 catalyst bank 2.

Why the warm-up catalyst fails first

It sits closest to the engine, which means it sees the highest temperatures and takes the full force of anything the engine does wrong.

Three things kill it:

Misfires. Unburnt fuel reaching a hot catalyst ignites inside it and can melt the substrate in minutes. This is why a flashing check engine lamp means stop driving: check engine blinking vs solid.

Rich running. Sustained over-fuelling overheats it and coats the substrate: P0172 and P0175 rich mixture.

Oil or coolant consumption. Contaminants coat the active surface and it stops working chemically, without any visible damage.

The cold-start dimension

Here is what makes P0421 different from P0422.

The warm-up catalyst’s job is largely finished within the first minute. Its efficiency test therefore runs during warm-up, when a lot of other things are also happening: enrichment, secondary air injection where fitted, and the oxygen sensors coming up to temperature.

That means P0421 can appear because of a cold-start behaviour problem rather than a dead converter:

• A slow-heating upstream oxygen sensor extends open loop and dumps fuel: P0030 to P0037 oxygen sensor heater.
• A failed secondary air injection system removes the heating assistance the catalyst was designed around: P0411 to P0414 secondary air injection.
• A coolant temperature sensor reading cold makes the engine over-enrich for longer: P0116 to P0119 coolant sensor.
• Short journeys that never let it reach temperature: short trips and engine damage.

Check all four before concluding the converter is finished.

What the sensors show

The catalyst monitor compares the upstream and downstream oxygen sensor traces. A healthy catalyst stores oxygen, so the downstream trace is much flatter than the upstream one.

Downstream trace	Meaning
Slow, small movements	Catalyst storing oxygen — healthy
Mirrors the upstream switching	No storage capacity left
Flat but offset high or low	Check the sensor before the catalyst

Log both together on one graph at a steady warm cruise. The relationship between them is the actual measurement.

O2 sensor waveform analysis.

Eliminate these before buying a converter

1. Exhaust leaks upstream of, or between, the sensors. A leak draws in air and skews the readings entirely.
2. A lazy downstream sensor, which cannot report storage it cannot detect: P0138 and P0140 downstream sensor.
3. Misfires, even ones below the threshold to set their own code. Check misfire counters per cylinder.
4. Fuel trims, which reveal a mixture problem feeding the converter.
5. Oil consumption, which will destroy a new catalyst as surely as it destroyed the old one.

That last point is the one that turns an expensive repair into a repeated expensive repair.

Aftermarket converters and this code

Non-original catalysts frequently set efficiency codes even when new, because the substrate loading is lower than the ECU’s threshold expects.

If you fitted an aftermarket unit and the code returned within a few hundred kilometres, that is usually the explanation rather than a second failure.

In what order

1. Read all codes — misfire, mixture and sensor codes reframe this one.
2. Check for exhaust leaks, particularly at the manifold.
3. Log both oxygen sensors at a warm cruise.
4. Investigate cold-start behaviour specifically for P0421.
5. Check oil consumption before replacing anything.
6. Fix the upstream cause first, then the converter.

Try it on the free tier of the app — pair an ELM327 adapter and log both oxygen sensors from a cold start through to full temperature. On a warm-up catalyst code, those first two minutes hold the answer.