enbgde
P0422 Generic (SAE)

Main Catalyst Efficiency Below Threshold (Bank 1)

Updated 2026-10-02 · Auxiliary emission controls

Severity
Medium
Typical repair cost
80 – 1 800 EUR
System
Powertrain

What it means

The engine control module compares the switching activity of the upstream and downstream oxygen sensors on bank 1 to judge how much oxygen the main catalyst stores. When the rear sensor starts mirroring the front sensor instead of holding a steady voltage, the module concludes the catalyst can no longer convert pollutants efficiently and sets P0422. On vehicles with a separate warm-up catalyst, this code targets the main underfloor unit rather than the close-coupled one.

Advertisement728 × 90

Symptoms

  • Check engine light on, often with no noticeable drivability change
  • Failed emissions test with high HC and NOx readings
  • Sulphur or rotten-egg smell from the exhaust under load
  • Slight loss of power or rattling from a disintegrating substrate
  • Marginally higher fuel consumption

Common causes

  1. Main catalyst worn out by age, mileage or contamination from oil or coolant
  2. Catalyst overheated by misfires or a rich mixture and melted internally
  3. Downstream oxygen sensor slow or biased, mimicking a weak catalyst
  4. Exhaust leak between the two sensors drawing in fresh air
  5. Unresolved misfire, injector or fuel trim fault poisoning the substrate
  6. Cheap aftermarket catalyst with insufficient precious-metal loading

How to diagnose

  1. Read all stored and pending codes first; fix any misfire, oxygen sensor or fuel trim codes before condemning the catalyst
  2. Inspect the exhaust from the manifold to the rear sensor for cracks, loose flanges and a damaged gasket; a leak upstream of the rear sensor can set this code on its own
  3. Graph both bank 1 oxygen sensors at 2500 rpm with the engine warm: the front sensor should switch 0.1-0.9 V several times a second while the rear sensor holds roughly 0.6-0.8 V with little movement
  4. Check long-term fuel trim; values beyond plus or minus 10 percent point to a mixture fault that will destroy a new catalyst
  5. Measure catalyst inlet and outlet temperature with an infrared thermometer after a 10-minute drive; the outlet should be at least 30-50 degrees C hotter than the inlet on a working unit
  6. Run the catalyst monitor with the scan tool and confirm the oxygen storage ratio reported; replace the catalyst only when the sensors, mixture and exhaust integrity have all been verified

Frequently asked questions

Can I keep driving with P0422?

Yes, the engine will run normally, but you will fail an emissions inspection and a disintegrating catalyst can eventually block the exhaust.

Will a catalyst cleaner additive fix it?

Rarely. Additives cannot restore worn precious-metal coatings; they help only when the substrate is lightly coated with carbon.

Why does the code return after a new catalyst?

Because the root cause, usually a misfire, rich mixture or oil consumption, was not fixed and has damaged the new unit or because a low-grade aftermarket catalyst was fitted.

Related codes

P0420P0430P0432P0141P0300

Nearby codes in this group