P0420 fault code: low catalyst efficiency (Bank 1)
Plain-language definition
Catalyst System Efficiency Below Threshold Bank 1
What does it mean?
The catalytic converter, often just called the catalyst or cat, turns harmful components of the exhaust gas into less harmful gases. The coating inside it keeps this conversion going by storing and releasing oxygen; as the part ages or gets damaged, this ability fades.
On petrol cars, the engine computer usually monitors the catalytic converter by comparing the signals of the oxygen (lambda) sensors in front of and behind it. Behind a healthy catalytic converter the sensor's signal stays steady; as efficiency drops, it starts to fluctuate much like the one in front. P0420 means the efficiency calculated for the Bank 1 catalytic converter has stayed below the threshold. Bank 1 is the side with cylinder 1; most single-bank engines have just one catalytic converter line.
Often there's no obvious difference while driving; the check engine light may be the only sign. On some cars there may be a rotten-egg smell from the exhaust; if the catalytic converter is clogged you may notice a loss of power, and if its insides have broken up, a rattle from the exhaust.
How serious is it?
This is a Warning-level code; it suggests the harmful gases coming out of the car may have increased. If misfire or fuel mixture codes are stored too, a new catalytic converter can also be damaged unless their cause is fixed.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- Aged catalytic converter, or one that has lost its efficiency
- Misfires or a rich mixture damaging the catalytic converter
- The engine burning oil or coolant
- Oxygen sensor after the catalytic converter giving wrong readings
- Exhaust leak near the oxygen sensors
- Replacement catalytic converter not suited to this engine
The list shows general causes; a workshop finds the actual cause on your car with a check.
Definitions by make
Some manufacturers record the same code under their own wording:
- Mitsubishi
- Catalyst System Efficiency Below Threshold (Bank 1); Catalyst deterioration
- Volkswagen
- Catalyst System Low Efficiency
Related codes
How does it appear in the 2dakika report?
Catalyst System Efficiency Below Threshold Bank 1
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.