P0B09: Electric/Auxiliary Transmission Fluid Pump A Motor Supply Voltage Circuit/Open
What to do
This is the version where the pump does nothing at all, so expect the car to be firmer about it — stop-start off, electric driving restricted, possibly a thump as it pulls away from rest. Ask for the supply to be checked first: a fuse, relay or connector is a fraction of the price of the pump, and the symptoms are identical.
What this code means
The supply feeding the auxiliary pump motor is missing or open — not one of the three phases the module drives, but the power rail those phases are made from.
The distinction is the whole of this code's value. A phase fault is a fault in one of three circuits and leaves the other two working; a supply fault means the pump has nothing to work with, so it does not run at all. Where the phase codes produce a pump that runs badly, this one produces a pump that is simply absent — and the transmission is left with no pressure whatever whenever the engine is off.
That makes it the more consequential half of the family and, usefully, the cheaper one to investigate: a supply is a fuse, a relay, a connector and a length of cable, all of which can be checked before anything is dismantled. Check the supply before condemning the pump, because a pump that is not being fed is indistinguishable from a pump that has failed, and only one of them costs a gearbox strip.
Likely causes
Start here
- Corroded or loose connector · easy
- Open circuit in pump wiring · moderate
Less common
- Damaged transmission wiring harness · moderate
- Faulty auxiliary transmission fluid pump · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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