P0B00: Electric/Auxiliary Transmission Fluid Pump Motor Phase U Current
What to do
The tell for this whole family is when it misbehaves: fine on the move, awkward from a standstill. If stop-start has quietly stopped working, or the car hesitates or thumps as it pulls away from rest, mention it — that pattern points here and a road test alone will miss it. The car disabling stop-start or electric driving is the cheap outcome; it is protecting the gearbox. Do not have it "coded out" to make the warning go away. Ask whether the pump's supply is high voltage on this model before anyone opens anything.
What this code means
The current in one phase of the auxiliary transmission fluid pump's motor is wrong. This is the first code of the P0Bxx range, and the pump it names exists for a reason worth understanding before any of the rest makes sense.
A conventional automatic gearbox gets its pressure from the engine. The fluid pump is driven through the torque converter, so pressure exists whenever the crankshaft turns — and the design quietly assumes the engine never stops while the car is in gear. A hybrid breaks that assumption at every set of traffic lights. The engine shuts down at rest, and during electric driving it may not run at all, yet the transmission still needs line pressure to hold its clutches applied and to be ready to move the instant the driver asks. So a second pump is fitted, driven by an electric motor rather than by the engine, and its whole job is to maintain pressure in the moments the engine is off.
That gives every code in this family its diagnostic signature: the fault is invisible while the engine is running. With the crankshaft turning, the main pump supplies everything and the car behaves perfectly. The symptoms appear only at the moments the auxiliary pump was supposed to cover — a delayed or harsh engagement pulling away from a stop, stop-start declining to work, electric-only driving refused. A driver reports "it only does it at the lights", and a road test with the engine running finds nothing at all.
The motor is three-phase, and the argument for that is already written at P0A5D: three identical circuits that must agree, four suspects when one stops matching, and a measurement that sorts them. One thing differs here and it matters for safety. The traction motor's phases are unambiguously high voltage; this pump's supply depends on the design — some are driven from the traction pack, others from a 12-volt or 48-volt rail. Establish which before anything is touched, and treat it as high voltage until you have.
Likely causes
Start here
- Faulty auxiliary transmission fluid pump · specialist
- Corroded or loose connector · easy
Less common
- Damaged transmission wiring harness · moderate
- Motor winding insulation breakdown · 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 P0B00.