P27E1: Electric/Auxiliary Transmission Fluid Pump A Pressure Too Low
What to do
The car will drive normally — the usual sign is stop-start no longer switching the engine off. That is deliberate: the module will not stop the engine when it cannot hold hydraulic pressure through the stop. Have it repaired rather than living with it, because on hybrids the same pump covers engine-off driving, and a shunt on every restart is wear the clutches were never designed to absorb.
What this code means
Electric auxiliary transmission fluid pump A is running but its pressure is too low.
This is the outcome code: the pump was commanded, it ran, and the pressure still did not arrive. The control circuit tests clean — what failed is the hydraulic result. That distinction matters, because it moves suspicion from the wiring to the pump's ability to actually move fluid.
Three things stop a running pump from making pressure. Worn pump elements let fluid slip back past them instead of being pushed forward; an internal leak — a tired seal, a perished O-ring — bleeds off what is built before it reaches the circuit; and fluid that is low, aerated or degraded gives the pump less to work with in the first place. The fluid check costs nothing and comes first.
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
- Low or degraded transmission fluid · moderate
Less common
- Perished seal or O-ring · specialist
- Hydraulic system leak · specialist
- Debris in the transmission fluid · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
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
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P27E1.