What Is P0420?
P0420 means the ECU has determined that the catalytic converter on Bank 1 is not converting exhaust gases efficiently enough. The ECU monitors catalyst efficiency by comparing the switching activity of the upstream (pre-cat) and downstream (post-cat) oxygen sensors. A healthy catalyst buffers exhaust gas variations so the downstream sensor voltage is relatively stable; a failed or degraded catalyst allows exhaust gas fluctuations to pass through, causing the downstream sensor to switch actively like the upstream sensor.
P0420 is very common on high-mileage vehicles — it is the most frequently stored fault code on many UK car models. The catalytic converter has a finite lifespan, typically 80,000–120,000 miles depending on engine condition and maintenance history. However, P0420 is also frequently triggered by a faulty downstream O2 sensor, exhaust leaks near the sensors, or underlying engine faults like misfires and oil burning that have poisoned the catalyst.
Common Symptoms
- Engine management light on
- Usually no driveability impact
- Possible slight reduction in fuel economy
- Possible sulphur smell from exhaust if catalyst is severely degraded
- Possible MoT failure on emissions test
Common Causes
How to Diagnose P0420
Check for Active Engine Faults First
Resolve any misfires, lean/rich codes, or coolant/oil burning before diagnosing P0420. Replacing a catalyst without fixing the underlying fault that destroyed it will result in the new catalyst failing quickly.
Inspect for Exhaust Leaks
Check the exhaust manifold and downpipe between the upstream O2 sensor and the catalyst, and between the catalyst and downstream sensor. A leaking gasket or cracked pipe can cause P0420 without any catalyst degradation.
Test the Downstream O2 Sensor
Compare downstream sensor switching activity to the upstream sensor in live data. Downstream should be relatively flat (0.6–0.8V steady). If downstream switches as rapidly as upstream, either the catalyst has failed or the downstream sensor is faulty.
Replace Downstream O2 Sensor First
A faulty downstream sensor is much cheaper to replace (£30–£80) than a catalytic converter (£200–£800). Replace it and retest before assuming the catalyst has failed.
Replace Catalytic Converter
If the downstream sensor is confirmed good and catalyst efficiency is still low, the converter requires replacement. Use a quality replacement — cheap catalysts fail quickly. Confirm the engine is in good health before fitting.
Mechanic's Corner — P0420 on UK Cars
Before spending £200–£600 on a replacement catalytic converter, test the upstream lambda (oxygen) sensor first. A slow or lazy upstream O2 sensor causes the ECU to misread combustion efficiency and incorrectly flag the catalytic converter as failing. This accounts for roughly 30% of P0420 cases in UK workshop experience. An upstream lambda sensor is typically £30–£80 and takes 20 minutes to replace versus a catalytic converter job that costs £250–£500 in parts alone.
Also worth knowing: P0420 will return even with a new catalytic converter if the underlying cause — typically running rich due to a faulty injector or coolant ingress — is not identified and resolved. A new cat fitted to an engine running rich will fail within 20,000 miles. If P0420 returns quickly after a cat replacement, begin with a cylinder compression test and injector balance test before spending more on parts.
Verdict
Always replace the downstream O2 sensor first and fix any existing engine faults before replacing the catalytic converter. A significant proportion of P0420 codes are caused by a faulty sensor rather than a dead catalyst. Catalyst replacement is expensive — confirm the sensor is not the issue first.
