P27A6: Electric/Auxiliary Transmission Fluid Pump B Stuck On
What to do
The car will drive normally — the risks are heat and a flat battery. A pump that will not switch off keeps drawing current after the engine stops, so a mysteriously flat 12-volt battery and this code are one problem, not two. Until it is repaired, expect the battery to need help sooner rather than later, and have the car looked at promptly.
What this code means
Electric auxiliary transmission fluid pump B keeps running when commanded off — stuck on.
Stuck on is the variant that costs you a battery. A pump that keeps running after it is commanded off draws current with the engine stopped — including overnight. It also churns fluid it no longer needs to move, which adds heat. A car with this code and a 12-volt battery that keeps going flat is one fault, not two.
This pump exists for the moments the engine is off. A conventional automatic makes its hydraulic pressure with a pump driven by the engine, so at a stop-start red light — engine stopped — pressure would drain away, and the first metre of drive would arrive with a thump while the mechanical pump refilled the circuit. The electric auxiliary pump holds pressure through those stops, so the clutches are already applied when the engine restarts. Hybrids lean on it even harder, because their engine stops whenever the battery can carry the load.
Which is why the commonest symptom is no symptom at all — except stop-start quietly giving up. The module knows it cannot hold pressure through an engine stop, so it stops allowing them. A car that used to switch off at the lights and no longer does, with this code stored, has already told you what is wrong. Where stops do still happen, the tell is a pause or a shunt as the car pulls away afterwards.
Likely causes
Start here
- Faulty auxiliary transmission fluid pump · specialist
- Debris in the transmission fluid · specialist
- Low or degraded transmission fluid · moderate
Less common
- Corroded or loose connector · specialist
Rare
- Faulty transmission control module · 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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