P0422: Main Catalyst Efficiency Below Threshold (Bank 1)

What it means

The engine control module has detected that the Bank 1 main catalyst system is below its monitored efficiency threshold. The code does not by itself identify the failed component.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The Bank 1 main catalyst treats exhaust after the engine has warmed, while the control module uses oxygen-sensor readings before and after it to estimate efficiency. P0422 sets when that comparison shows the Bank 1 main catalyst below its monitored threshold; the code alone does not prove the catalyst itself is the cause.

Common causes

Most likely first.

1. Catalyst aged or contaminated
2. Oxygen sensor drift

Symptoms

• Check engine light on
• Failed emissions inspection
• Possibly slightly increased fuel consumption
• Sulfur (rotten egg) smell from exhaust under load

How to diagnose it

1. Review freeze-frame data and check for Bank 1 misfire, fuel-trim, exhaust-leak, or oxygen-sensor codes before judging the catalyst.
2. Inspect the Bank 1 exhaust joints and oxygen-sensor wiring for leaks or damage that could distort readings.
3. Compare upstream and downstream Bank 1 oxygen-sensor live data at operating temperature; a downstream pattern that closely tracks upstream can indicate low catalyst oxygen storage.
4. Check the oxygen sensors for slow, biased, or implausible readings and verify their wiring and connections.
5. If other engine and sensor faults are ruled out, test the Bank 1 main catalyst using the manufacturer's diagnostic procedure.

Common fixes

• Repair exhaust leaks or oxygen-sensor wiring faults on Bank 1.
• Fix any misfire or fueling problem that may have contaminated the catalyst.
• Replace a confirmed faulty Bank 1 oxygen sensor.
• Replace the Bank 1 main catalyst if testing confirms it is below the efficiency threshold.

Can I keep driving?

If the engine runs normally, driving gently to arrange service is generally possible, but the vehicle is likely to fail an emissions inspection. Stop and investigate promptly if power falls, the exhaust smell becomes strong, or the engine misfires, since catalyst damage can worsen.