P0939: Hydraulic Oil Temperature Sensor Circuit Low
What to do
The gearbox may slow or refuse changes as though overheating. If that happens after only a short, gentle drive, treat the reading with more suspicion than the gearbox — a sensor shorted to ground looks exactly like fluid that is far too hot.
What this code means
The hydraulic fluid temperature circuit reads below its valid voltage range, normally a short to ground. These are thermistors whose resistance falls as temperature rises, so a short to ground is electrically identical to impossibly hot fluid — and the module protects the system accordingly, slowing or refusing gear changes to let a heat problem that does not exist subside.
As with P0712 on an automatic gearbox, the fault manufactures its own alarming symptoms: a car that behaves as though it is overheating after a gentle five-minute drive is more likely to have a shorted sensor than a genuine problem.
Likely causes
Start here
- Wiring shorted to ground · moderate
- Faulty hydraulic oil temperature sensor · moderate
Less common
- Corroded transmission connector · moderate
- Fluid tracking into the transmission connector · moderate
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 P0939.