P0422: Main Catalyst Efficiency Below Threshold (Bank 1)

What is the engine computer reporting with P0422?

This code applies specifically to the main underfloor catalytic converter rather than a smaller pre-catalyst, and sets on the same basis as the general catalyst efficiency code, a downstream oxygen sensor pattern that has started to mirror the upstream sensor instead of showing a converter buffering oxygen normally. On engines with both a warm up and a main catalyst, this code narrows the fault to the second, larger converter.

The code sits in the catalytic converter efficiency area and is reported by the catalyst monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Failed emissions test
• Check engine light with no other symptom
• Slightly reduced fuel economy
• No noticeable change in how the car drives

What is actually behind P0422?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a catalytic converter efficiency fault becoming an expensive guess.

Downstream oxygen sensor for the main catalyst giving a false reading is the one to rule out before anything else. A cheap sensor fault can mimic a failing converter and should be ruled out first.

How do you diagnose P0422?

Start with freeze frame. Engine load and fuel trim values at the fault show whether the engine was running rich or lean at the time. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

This code is diagnosed by watching values change rather than by reading the number again, so the tool has to stream live data and hold freeze frame. Live upstream and downstream oxygen sensor waveforms for the main converter, graphed together over a drive cycle, show whether it is actually buffering oxygen correctly.

The order worth working in

• 1. Downstream oxygen sensor for the main catalyst giving a false reading: A cheap sensor fault can mimic a failing converter and should be ruled out first.
• 2. Exhaust leak between the warm up catalyst and the main catalyst: A leak in this section skews the sensor comparison used to judge the main converter.
• 3. Rich or lean running condition from an unrelated fault overloading the main catalyst: A converter exposed to a long-term rich or lean condition wears out faster.
• 4. Main catalytic converter genuinely degraded with age or mileage: This is the correct diagnosis only after the cheaper upstream causes are ruled out.
• 5. Contamination from oil consumption or coolant leaking into a cylinder: Contamination poisons the catalyst coating permanently and cannot be cleaned or reversed.

Can you keep driving with P0422?

Safe to keep driving, since this reflects worn emissions control rather than a mechanical risk to the engine.

This is a severity 2 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0422?

A genuinely worn main converter will eventually fail an emissions test, and if the underlying rich, lean or misfire condition that damaged it is never fixed, a replacement converter fails again.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.