OBD-II trouble code
P0945: Hydraulic Pump Relay Circuit/Open
An open in the low-current circuit that switches the hydraulic pump on. Cheap part, expensive consequences — and when it arrives alongside the pressure codes, it is usually their cause rather than their companion.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $900
- DIY difficulty
- Advanced DIY
Browse every code in P0900–P0999, or start from the full code library.
What does P0945 mean?
P0945 is the odd one out in this group, and in a useful way. Every other code around it describes hydraulics. This one describes a switch.
The pump motor that pressurises the transmission's actuator circuit draws far more current than a control module can supply directly, so the module does not power it. It energises a relay coil with a fraction of an amp, and the relay's contacts carry the real load from the battery to the motor. P0945 is set when the module finds that low-current control side open — the coil circuit is broken, the driver output cannot see its expected load, or the relay is simply not there electrically. The code is about the control circuit, not about the pump.
The reading that matters most is what this code means when it does not arrive alone. A pump that never runs cannot build pressure, and a system that has no pressure will produce exactly the pressure-unit faults that sit at P0942 and P0944. So when P0945 appears together with them, the sensible order of work is to settle the relay first and treat the rest as honest reporting of a real absence rather than as separate faults. Getting that backwards is how a twenty-dollar relay becomes a quoted hydraulic assembly: the expensive codes are louder, the expensive part is the obvious suspect, and nothing in the code list flags which one came first. Clearing the codes, fixing the relay circuit and seeing what actually returns is a cheap experiment and the right first move.
The circuit itself is small and testable, and the arithmetic gives it away quickly. The coil has a specified resistance and should draw a predictable few tens of milliamps; an open coil reads infinite and draws nothing. On the load side, a working pump motor pulls a substantial and measurable current from its supply, so clamping the feed or reading the current at the fuse during a commanded pump run tells you in one measurement whether the contacts are actually closing. Zero current with the relay commanded on is a relay or control-side problem. Normal current with no pressure appearing moves the investigation back into the hydraulics, where the other codes in this block live.
Where the labour cost varies is packaging. On some vehicles this is a conventional plug-in relay in an engine bay box, replaceable in two minutes for the price of a coffee. On others the same function lives inside a sealed fuse and relay module, or has been absorbed into the transmission control unit as a solid-state driver, in which case there is no relay to pull and the repair becomes a module. That is the difference between a twelve-dollar part and a four-figure one, and it is worth establishing which design the vehicle has before anybody quotes anything.
A final note on how the car behaves. An open control circuit that is genuinely permanent usually leaves the vehicle unable to drive at all, which puts it into P0944 territory. If the car is still driving with P0945 stored, the pump is evidently still running at least some of the time — which points at an intermittent open, a relay with tired contacts or a connection that is failing under vibration or heat rather than a clean break. That is a useful thing to know, because it means the fault is likely to be found by disturbing things rather than by measuring a circuit that currently reads correctly.
Common causes
- Relay coil failed open internally
- Blown fuse in the relay control or supply circuit
- Corroded or spread terminals in the relay socket
- Open in the control wire between the module and the relay coil
- Poor ground on the relay coil circuit
- Relay not fully seated in its socket after previous work
- Water ingress into the fuse and relay box
- Heat-damaged socket terminals from prolonged high pump duty
- Failed solid-state pump driver inside the transmission control module
- Wiring damage where the loom runs to the pump or relay box
Symptoms
- Hydraulic pump silent when it should be running
- Transmission slow to build pressure or failing to select from cold
- Check engine light with multiple transmission pressure codes stored together
- Loss of drive that recovers after switching the ignition off and on
- Intermittent transmission warning under vibration or on rough roads
- Pump running normally on some journeys and not at all on others
- Limp mode engaging without any mechanical noise from the transmission
- Fault appearing after work in the engine bay fuse and relay box
- Complaint that worsens as the engine bay heats up
- Complete inability to select a gear in the worst cases
Diagnostic steps
- 1.Read all stored codes together and treat P0945 as the likely cause when pressure-unit codes are present alongside it, rather than as a separate fault.
- 2.Check the relevant fuses on both the control and load sides before testing anything else.
- 3.Establish whether this vehicle uses a serviceable plug-in relay, a sealed relay module or a solid-state driver inside the control unit, because that decides the whole cost of the repair.
- 4.Measure the relay coil resistance and compare it against specification — an open coil reads infinite.
- 5.Measure current in the pump supply during a commanded pump run: zero current means the contacts are not closing, normal current moves the investigation into the hydraulics.
- 6.Test the control wire from the module to the relay coil for continuity end to end.
- 7.Voltage-drop the relay coil ground rather than checking it for continuity alone.
- 8.Inspect the relay socket for corroded, spread or heat-darkened terminals, which are common after a long period of high pump duty.
- 9.Check the fuse and relay box for water ingress, particularly on vehicles where it sits low or near a scuttle drain.
- 10.Clear the codes after repairing the relay circuit and re-test before accepting that any hydraulic fault is real.
Repair cost
$90 – $900
Diagnosis is $90 to $220. A plug-in relay is $12 to $60 for the part and often under half an hour, so $90 to $180 all in is the common outcome. A socket, connector or control-wire repair is $120 to $400. Where the function lives inside a sealed fuse and relay module the assembly is $200 to $600 fitted. Where it is a solid-state driver inside the transmission control unit there is no relay to replace and the repair becomes a module replacement with programming, which is a separate and much larger job. Establish which design the vehicle has before approving anything.
Estimate your repair
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Open the Repair Cost Estimator with hydraulic pump relay replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.