P0420
Catalyst System Efficiency Below Threshold (Bank 1)
The oxygen storage capacity of the main catalytic converter on bank 1 has degraded below the OBD threshold. The downstream oxygen sensor signal increasingly mirrors the upstream sensor, indicating reduced conversion efficiency.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
What we see in the workshop
ON FORD ECOBOOST
Catalyst efficiency below threshold is often the downstream sensor or an upstream misfire that has damaged the cat, rather than a failed converter. On cars fitted with an aftermarket downpipe this code is expected, and we will tell you so rather than sell you a catalytic converter.
ON ALFA ROMEO
We check for an upstream misfire or oil consumption before condemning the converter.
Symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Common causes
Ordered by how often each turns out to be the cause.
- Catalyst aged or contaminatedCommon
- Downstream oxygen sensor driftPossible
- Exhaust leak before catalystLess likely
- Long-term misfire damageLess likely
Parts involved
- Catalytic Converter
- Oxygen Sensor Upstream
- Oxygen Sensor Downstream
P0420 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.