P0422

What does P0422 mean?

P0422 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Main Catalyst Efficiency Below Threshold (Bank 1).

This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.

Symptoms commonly associated with this code

Symptoms that drivers often report alongside this code. Not all may apply to every case:

• • Engine warning light on, frequently the only thing you'll notice day to day
• • Slightly worse fuel economy, usually obvious only over a tank or two of motorway driving
• • A faint sulphur or rotten-egg smell from the back of the car under load
• • On a partially blocked cat, the engine feels flat and slow to pull away
• • If the substrate has broken up inside, a rattle from under the floor when you blip the throttle or go over bumps
• • Failed emissions readings if you take it for an MOT with the cat gone

Possible causes

Causes commonly associated with P0422, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.

1. 1. Worn-out or contaminated main cat, easily the top cause once a car is past 90,000 miles and has been through plenty of heat cycles
2. 2. Lazy or faulty rear oxygen sensor reporting rubbish, the cat is fine but the ECU is being lied to
3. 3. Misfires dumping raw fuel into the exhaust, which cooks the substrate fast. Sort any misfire before you touch the cat or you'll wreck the new one
4. 4. Engine running rich over a long period, often a dud coolant temp sensor, a weeping injector or high fuel pressure
5. 5. Exhaust leak ahead of the rear sensor pulling fresh air into the reading and faking a failed cat
6. 6. Aftermarket decat or cheap sports cat fitted, in which case the code is a side effect but it'll still fail the MOT
7. 7. Coolant getting into the cat from a head gasket fault, which coats and poisons the substrate

How mechanics typically diagnose

A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.

1. 1. Read live data from both oxygen sensors first. A good cat shows the rear sensor sitting fairly flat, around 0.6 to 0.8V, while the front one swings up and down quickly. If the rear is swinging as much as the front, the cat really is on its way out
2. 2. Pull every stored code, not just the P0422. Misfire codes in the P0300 range or lean trims like P0171 nearly always need fixing before you condemn the cat, otherwise you'll kill the replacement
3. 3. Get under the car and look hard at the exhaust for cracks, blown gaskets or a rotten section before the rear sensor. A small leak there will throw this code with a perfectly healthy cat
4. 4. Check long-term fuel trim. If it's sitting well below zero, say worse than -15%, the engine has been running rich and that's what's been damaging the cat
5. 5. Swap in a known-good rear oxygen sensor if you can get hold of one. High-mileage cars often just have a tired sensor and this is the cheap win
6. 6. If sensors, trims and the exhaust all check out, the cat itself is the problem and you're into deciding whether the car's worth the spend

Common questions about P0422