Check Engine Light (P0420)? How to Clean a Catalytic Converter Without Replacement
A failed catalytic converter is one of the most dreaded diagnoses for any car owner. With replacement costs often ranging from £400 to over £1,000, it’s a repair bill nobody wants. But before you book your car in for a new exhaust, you should know that many “failed” converters are simply dirty.
Common issues like short trips, poor quality fuel, or oil burning can coat the catalyst honeycomb in carbon, triggering the dreaded Check Engine Light (usually fault code P0420 or P0430). In this guide, we’ll explain how to use XADO Red CAT Shield to clean the system, restore performance, and pass your emissions test for a fraction of the cost.
What Does the Catalytic Converter Do?
The catalytic converter is a vital part of your exhaust system. Its job is to turn harmful pollutants (Carbon Monoxide, Hydrocarbons, and Nitrogen Oxides) into harmless gases before they leave the tailpipe.
Inside the metal shell is a ceramic honeycomb coated with precious metals (platinum, palladium, rhodium). These metals trigger a chemical reaction to clean the exhaust. If this honeycomb gets coated in soot, oil, or unburnt fuel, the chemical reaction slows down and your car’s ECU triggers a warning light.
Why Catalytic Converters Fail
A catalytic converter rarely fails on its own. Most issues are caused by upstream problems such as:
- Short journeys that never heat the catalyst fully
- Oil burning from worn piston rings or valve seals — see our engine oil additives
- Rich fuel mixture caused by faulty sensors or misfires
- Poor quality fuel leaving carbon deposits — explore our fuel system cleaners
When these contaminants coat the honeycomb, the converter becomes “inefficient” — triggering fault code P0420.
What P0420 / P0430 Actually Mean
P0420 (Bank 1) and P0430 (Bank 2) mean the catalytic converter is not cleaning exhaust gases as efficiently as expected. This is usually detected by comparing the readings of the upstream and downstream oxygen sensors.
It does not automatically mean the converter is broken — only that its efficiency is below the ECU’s threshold.
Symptoms of a Dirty or Clogged Catalytic Converter
Aside from the dashboard warning light, watch out for these physical signs:
- "Rotten Egg" Smell: A strong sulphur odour from the exhaust is a classic sign of catalyst contamination.
- Loss of Power: If the converter is clogged, exhaust gases can’t escape, choking the engine.
- High Fuel Consumption: The engine works harder to push exhaust out.
- Failed MOT Emissions: especially on High CO (Carbon Monoxide) or Lambda readings.
Recommended Product
XADO Red CAT Shield (250ml) — Cleans the catalytic converter, oxygen sensors, and fuel system. Extinguishes P0420 Check Engine Lights caused by contamination.
Can You Clean a Catalytic Converter?
Yes — but it depends on the type of failure. You cannot fix a converter that has physically melted or broken apart (listen for a rattling noise when you tap the exhaust). However, if the failure is due to chemical poisoning or carbon deposits, a specialised cleaner can restore efficiency.
How to Use XADO Red CAT Shield (Step-by-Step)
Unlike simple fuel additives, catalytic converter cleaners need to run through the combustion process at a specific concentration.
- Wait for Low Fuel: Wait until your fuel tank is low (approx. 20 litres remaining).
- Add the Bottle: Pour the entire 250ml bottle of XADO Red CAT Shield into the fuel tank.
- Drive: Drive normally until the tank is nearly empty. The cleaning agents survive combustion and clean the exhaust system as they pass through.
- Refuel: Fill up with high-quality petrol.
For best results, include a motorway drive (20–30 minutes) to get the catalytic converter up to optimal operating temperature.
When Will a Cleaner Work? (The Reality Check)
It is important to diagnose the issue correctly. Here is a quick checklist:
| Cleaner WILL Help | Cleaner WILL NOT Help |
|---|---|
| ✔ Code P0420 / P0430 (Efficiency Low) | ✖ Rattling noise from exhaust (Broken internals) |
| ✔ "Rotten Egg" smell | ✖ Converter glowing red hot (Melted) |
| ✔ Failed Emissions (High CO) | ✖ Exhaust leaking/blowing (Needs welding) |
| ✔ Loss of power / sluggishness | ✖ Faulty Oxygen (O2) Sensor (Needs replacement) |
Catalytic Converters and MOT Emissions
If you are failing your MOT on emissions, the catalytic converter is often the prime suspect. However, a dirty converter is often a symptom of another problem, such as oil burning or misfires.
Using XADO Red CAT Shield before your MOT cleans the O2 sensors as well as the catalyst. Clean sensors send more accurate data to the ECU, optimising the fuel mixture and giving you the best chance of passing.
For more tips on passing the emissions test, read our guide on how to pass MOT emissions first time.
For a simple overview of catalytic converter cleaners, see our introductory guide: 5 Things to Know About Catalytic Converter Cleaners.
External references for further reading:
Before You Replace Your Catalytic Converter…
Check these items first:
- Is the engine burning oil? — see our oil burning solutions
- Are there any misfire codes?
- Are the O2 sensors old or slow to respond?
- Is the exhaust leaking before the converter?
- Has the car been used mainly for short trips?
Don't Replace Your Cat Before You Try This
A new catalytic converter costs hundreds. XADO Red CAT Shield costs a fraction of the price and cleans the entire exhaust system while you drive. Ideal for extinguishing P0420 engine lights and fixing rotten egg smells.