P04F4
Catalyst Heater Circuit Low Bank 1
The control or supply line to the electric catalyst heater on bank 1 is shorted to ground or measures below the expected voltage when the heater is commanded on.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
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.
- Control line shorted to groundCommon
- Heater element internally shortedPossible
- Connector pins corroded - high contact resistanceLess likely
Parts involved
- Electric Catalyst Heater B1
- Catalyst Heater Wiring B1
P04F4 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.