Exhaust and emissions
What is a catalytic converter and why does it fail?
The catalytic converter is the part of the exhaust that turns the most polluting gases into less harmful ones. The control unit compares the oxygen sensor before and after it: if the downstream sensor swings almost like the upstream one, the converter no longer stores oxygen and a code such as P0420 is stored. Before replacing it, rule out the sensor, exhaust leaks and misfires.
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What exactly does it do?
On gasoline engines, the three-way catalytic converter reduces carbon monoxide, unburned hydrocarbons and nitrogen oxides at the same time. On diesels, the oxidation catalyst handles carbon monoxide and hydrocarbons; NOx is handled by the EGR or by SCR with DEF (AdBlue).
What ruins it?
Misfires: unburned fuel burns inside the converter and overheats it. That is why, with a flashing check engine light, you should ease off and stop as soon as possible.
An engine that burns oil or coolant contaminates it, and an impact can break its core. With high mileage it also loses efficiency through wear.
What to check before replacing it?
That there are no misfires or a rich mixture, that the downstream oxygen sensor works and that there are no exhaust leaks between the two sensors.
If it has to be replaced, the new one must be approved for your vehicle. Removing or bypassing it is not allowed and the car will fail inspection.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0420Catalyst system (bank 1): efficiency below threshold
- P0430Catalyst system (bank 2): efficiency below threshold
- P0300Random or multiple cylinder misfire detected
- P0136O2 sensor (bank 1, sensor 2), circuit: malfunction
Related symptoms
- Rotten egg smell from the car: what it means
- Flashing engine warning light: what it means and what to do
- Engine warning light on and steady: what it means and what to do
Learn more
See what your car is saying
With the ECO-LITE you see the code and the catalyst monitor status, and your workshop can review the oxygen sensors live, remotely.
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Frequently asked questions
Can I keep driving with a P0420?
Usually yes in the short term if the car runs fine, but it will not pass an emissions inspection and you should know the cause.
Does an additive fix the catalytic converter?
There is no guarantee. If the converter is damaged, an additive will not bring it back; find the cause first.
What does a rotten egg smell mean?
It usually points to a rich mixture or a converter that is not working well: have it checked.