Catalytic Converter Diagnosis
Also called catalyst efficiency or P0420 check
This is a diagnosis, not a repair. The catalytic converter cleans exhaust gas, and when it fails the car sets an efficiency fault code, loses power, or in bad cases blocks the exhaust so the engine cannot breathe. Testing compares oxygen sensor readings before and after the converter and measures back pressure. It matters because the same fault code is often caused by a cheap oxygen sensor or an upstream leak rather than the converter itself.
What the job includes
- Reading and recording stored fault codes and freeze frame data
- Comparing live readings from the sensors before and after the converter
- Measuring exhaust back pressure to check for a blockage
- Checking for upstream exhaust leaks that imitate converter failure
- A written finding stating whether the converter itself is at fault
Signs you need this
- Check engine light with a catalyst efficiency code
- Car feels breathless and will not pull above a certain speed
- Smell of sulphur or rotten eggs from the exhaust
- Rattling from the converter body when tapped
- Fuel consumption climbing with no other explanation
Not all of these have to be present, and one of them on its own can mean something else entirely. That is what the diagnosis is for.
How we do it
Read the evidence
Stored codes and freeze frame data tell us the conditions under which the fault set, which narrows the search considerably.
Compare sensor signals
We watch both oxygen sensors on live data at held engine speeds to see whether the converter is still working.
Test for restriction
Back pressure is measured at a sensor port to show whether the converter has partly collapsed and is choking the engine.
Report the finding
You get a plain explanation of what the tests showed and what the honest options are, including doing nothing for now.
How we work
The same on every job, which is why we can write it down.
Diagnosis before quote
We find the fault first. A price given over the phone for a noise nobody has heard is a guess.
The price is agreed first
You approve the work and the cost before a spanner is picked up. Extra work means another call.
Parts you can check
We tell you whether a part is genuine, OEM, aftermarket or used, and what each one costs.
Old parts returned
Ask and we will hand back whatever we replaced. It is the simplest proof that the job was done.
Book catalytic converter diagnosis
Fill this in and a person will call you back to confirm the time and the price. Only your name and one way to reach you are required.
Before you book
Questions we get asked
If your question is not here, ask it. We would rather answer honestly now than have you find out after booking.