P0421

What it means

P0421 means the engine control unit has judged that the warm-up catalytic converter on bank 1 is not storing and converting oxygen as well as it should. It reaches this view by comparing the signal from the sensor before the catalyst with the one after it. A healthy catalyst gives a steady, slow-moving rear signal, whereas a worn one lets the rear signal mimic the front. The code is set once the test fails over one or more monitoring cycles.

Symptoms

• Engine warning light on
• Sulphur or rotten egg smell from the exhaust
• Slightly higher fuel consumption
• Reduced power or sluggish acceleration in some cases
• Failed emissions test at the MOT
• Often no noticeable change in driveability

Possible causes

• Worn or contaminated catalytic converter on bank 1 (age, mileage, thermal damage or contamination)likely
• Engine misfire or rich or lean running that has overheated or fouled the catalystlikely
• Faulty or slow downstream oxygen sensor on bank 1 giving a false readingpossible
• Exhaust leak before or around the downstream sensor, letting air skew the readingspossible
• Damaged wiring or connector to the upstream or downstream oxygen sensorpossible
• Oil or coolant burning in the engine, contaminating the catalystpossible
• Outdated engine control software where a known fault with the catalyst monitor exists for that modelrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, and note any misfire, fuel trim or oxygen sensor codes that need dealing with first
2. Inspect the exhaust from the manifold to the rear sensor for leaks, damage and loose joints, and check the sensor wiring and connectors
3. Check for signs of misfire, oil or coolant consumption and look at the spark plugs for fouling
4. Review long and short term fuel trims at idle and at 2,500 rpm to spot rich or lean running
5. Watch live upstream and downstream oxygen sensor signals at a steady warm cruise; a downstream signal that swings in step with the upstream one points to a tired catalyst
6. Check the sensors' response and heater operation to rule out a lazy or failed downstream sensor
7. Check the catalyst for overheating, rattling or restriction, and measure exhaust back pressure if a blockage is suspected
8. Look up any manufacturer service information or software updates for the specific model and engine

How to fix

• Repair any exhaust leak found near the catalyst or sensors
• Cure misfires, fuel mixture faults or oil and coolant consumption before fitting anything new
• Replace a faulty downstream or upstream oxygen sensor, or repair its wiring and connector
• Replace the bank 1 catalytic converter with a suitable type-approved unit once other causes are ruled out
• Apply an updated engine control software calibration only where the manufacturer lists it as a fix for this fault

Can I keep driving?

Nearby codes

• P0420GenericCatalyst System Efficiency Below Threshold Bank 1
• P0422GenericCatalyst 2 Efficiency Below Threshold Bank 1