P0420 Fault Code on Honda Civic — UK Guide (2005–2017)

P0420 on the Honda Civic — what you need to know

Honda Civic P0420 is commonly caused by a failing downstream O2 sensor. Honda catalysts are generally very durable — if the sensor tests fine, inspect for exhaust leaks before replacing the catalyst.

The affected engine variants are: 1.4, 1.6, 1.8 i-VTEC, 2.0 i-VTEC, 2.2 i-CTDi. If your Honda Civic is showing fault code P0420, use our AI Diagnostic Report for a full UK-specific analysis including the exact repair costs for your engine and year.

Common causes on the Honda Civic

While P0420 can have several root causes, the most common reasons this code appears on the Honda Civic in UK conditions are related to the specific engine management and emissions systems fitted to this vehicle. Always scan for related fault codes before replacing any parts — a secondary code can point directly to the root cause and save hundreds of pounds.

For the complete diagnosis guide including step-by-step testing procedures, see our main P0420 fault code page.

UK repair costs for P0420 on the Honda Civic

Typical UK repair costs for P0420 on the Honda Civic range from £150 to £800 depending on the exact cause, your engine variant and whether you use an independent garage or a main dealer. Independent garages are typically 30–50% cheaper than franchised dealers for this type of repair.

Always get at least two quotes. Use our AI Diagnostic Report to understand the most likely cause for your specific Honda Civic before spending anything at a garage.

P0420 MOT implications for the Honda Civic

An active P0420 fault code on your Honda Civic is likely to cause an MOT failure. Under DVSA testing standards, an illuminated check engine light is an automatic MOT failure regardless of how the vehicle drives. You must resolve this fault before your MOT test. Read our full MOT guide for more information.