P0420: Catalyst System Efficiency Below Threshold (Bank 1)

Can I keep driving?

What to do right now

1. 1Check your instrument cluster to confirm the amber engine light is steady and not flashing.
2. 2Listen carefully for any unusual hissing, blowing, or rattling noises coming from beneath the vehicle floor.
3. 3Continue your journey with moderate acceleration and arrange to have the car inspected soon.

Likely causes

Most likely first.

1. Likely

   Worn or degraded catalytic converter

   Over time and mileage, the precious metal coatings inside the converter break down and lose their ability to neutralise harmful exhaust gases.

2. Possible

   Exhaust leak near the rear sensor

   A cracked pipe, split weld, or blown gasket draws fresh air into the exhaust stream, tricking the downstream sensor into reporting poor catalyst performance.

3. Possible

   Faulty downstream oxygen sensor

   The oxygen sensor fitted after the converter can become slow or provide inaccurate voltage signals as it ages, prompting the engine computer to log an efficiency fault.

4. Possible

   Unburnt fuel or oil entering the exhaust

   A lingering engine misfire or burning engine oil can coat the converter internals in soot or overheat the core, degrading its cleaning efficiency.

What you can check yourself

Park somewhere safe and level, engine off, before you start.

1. 1

   Check warning light behaviour

   Note whether the engine light stays steady or flashes while driving. A steady light means you can drive on cautiously, but a flashing light warns of an active misfire that can melt the converter quickly.

2. 2

   Listen for exhaust blows or rattles

   Park on a flat surface, apply the handbrake, and let the engine idle while you stand beside the car. A loud puffing or ticking sound suggests an exhaust leak, whereas a metallic rattle under the floor can indicate a broken ceramic core inside the converter.

3. 3

   Scan for companion fault codes

   Plug a handheld OBD scanner into the port beneath the steering wheel to check for extra codes. Codes relating to engine misfires, fuel trim, or oxygen sensor heaters must always be repaired before replacing a converter.

4. 4

   Inspect engine oil level and exhaust colour

   With the engine cold and switched off, pull the dipstick under the bonnet to verify oil level. If the level drops quickly between services or you spot blue smoke from the exhaust, burning oil may be poisoning the catalyst.

Typical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The final bill depends heavily on whether the catalytic converter itself has failed or if a simple sensor or exhaust gasket leak triggered the code, with genuine manufacturer parts costing substantially more.

When to go to a garage

• The engine management light starts flashing, which signals an active engine misfire that can rapidly melt the catalytic converter.
• The car noticeably loses power, hesitates during acceleration, or refuses to rev freely, suggesting a blocked exhaust.
• You smell strong exhaust fumes or sulphur inside the vehicle cabin while driving.
• The warning light returns after clearing the code, confirming an active emissions fault that requires garage diagnosis.

Questions drivers ask

Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.