OBD-II trouble code
P0947: Hydraulic Pump Relay Circuit Low
Low voltage on the pump relay circuit. The part is cheap, but the useful question is why it wore out — because if the pump has been short-cycling, a new relay is on the same clock the old one was.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $1,500
- DIY difficulty
- Intermediate DIY
Browse every code in P0900–P0999, or start from the full code library.
What does P0947 mean?
P0947 is stored when the module sees the pump relay circuit sitting at a voltage below the band it expects. Depending on which side of the coil the module drives and where it takes its measurement, that can mean a short to ground on the control wire, a coil that has partly failed, or contacts whose resistance has climbed high enough that the voltage reaching the pump is no longer what it should be.
Replacing the relay will usually clear it. This page is about the part that gets skipped afterwards, because on this particular circuit the relay is rarely the first thing that went wrong — it is usually the second.
A relay driving a motor does not fail on a calendar. It fails on a count of switching events, and specifically on the current flowing at the instant the contacts close. An electric motor at rest is very nearly a short circuit: with no rotation there is no back-EMF opposing the supply, so the first surge through the windings is many times the current the motor settles to once it is spinning. That inrush is what pits and erodes contact faces. A relay that closes onto a stalled motor a few times an hour will outlive the car. The same relay closing every ninety seconds will not.
That is the link worth carrying away, and it points at a code in the same block. P0943 exists because a hydraulic pump is meant to run in bursts — it charges an accumulator, stops, and stays off until pressure decays. When a seal, an accumulator precharge or an internal leak lets pressure fall faster than it should, the pump is recalled far more often than the design intended. Every one of those extra starts is another inrush event on the relay. So a car with a history of short-cycling is a car whose pump relay is being consumed at several times its intended rate, and the two codes are the same story told from opposite ends of the circuit.
Which gives the diagnosis a second question. Not only 'is the relay bad', but 'how often is this pump starting'. A scan tool that reports pump duty cycle or run counts answers it directly; where that data is not available, listening to how long the pump stays off after a full charge does the same job more slowly. A pump that rests for minutes is healthy. A pump that restarts within a few tens of seconds on a stationary car is telling you that the replacement relay you are about to fit will be back.
The wear pattern also explains why this fault so often arrives gradually. Contact resistance climbs before it fails outright, so the pump runs slightly slower and pressure builds slightly later for weeks before any code appears. Owners describe a car that has become fractionally lazy to engage from cold and then, one morning, will not engage at all. That slow shoulder is characteristic of a wear fault and is a reason to voltage-drop the closed contacts under load rather than trusting a relay that still clicks.
Common causes
- Relay contact faces eroded by repeated motor inrush current
- Short to ground on the relay coil control wire
- Partially failed relay coil with reduced resistance
- High resistance across worn contacts reducing voltage at the pump
- Chafed control wire grounding against the fuse box bracket or body
- Corroded relay socket terminals adding resistance in series
- Excessive pump start frequency caused by an underlying hydraulic leak
- Water intrusion into the relay box creating a low-resistance path
- Failed low-side driver inside the transmission control module
- Poor ground on the pump motor circuit dragging the measured voltage down
Symptoms
- Transmission slow to engage from cold, worsening over several weeks
- Pump audibly running longer than it used to for the same result
- Pump restarting far more frequently than expected on a stationary car
- Delayed or missed gear selection followed by normal operation once warm
- Check engine light with pressure-related transmission codes stored alongside
- Intermittent limp mode that clears on an ignition cycle
- Relay clicking normally but the pump sounding weak or slow
- Repeat relay failures at short intervals after previous replacement
- Fault appearing shortly after the car has stood unused for a long period
- Complaint that is worse on the first start of the day
Diagnostic steps
- 1.Read all stored codes and note specifically whether a short-cycling code such as P0943 is present now or in history — that changes the repair.
- 2.Check pump run counts or duty cycle on the scan tool, or time how long the pump stays off after a full pressure charge on a stationary car.
- 3.Measure the relay coil resistance and compare with specification; a low reading indicates partial shorting inside the coil.
- 4.Disconnect the module connector and check the coil control wire for a short to ground.
- 5.Voltage-drop the closed relay contacts while the pump is running rather than judging the relay by whether it clicks.
- 6.Measure supply voltage at the pump motor itself during a run and compare it with battery voltage.
- 7.Inspect the relay socket terminals for corrosion, discolouration and spread, all of which add series resistance.
- 8.Follow the control wire along its route and inspect where it passes brackets or edges for chafing.
- 9.Ask the owner whether the car has recently been laid up, since long standing periods cause corrosion faults that behave like this one.
- 10.If a relay has already been replaced once and failed again quickly, stop replacing relays and investigate why the pump is starting so often.
Repair cost
$90 – $1,500
Diagnosis is $90 to $220. A plug-in relay is $12 to $60 and typically under half an hour, so $90 to $200 covers most straightforward cases. Socket or control-wire repair adds $120 to $400. The number that catches people out is the underlying cause: if the pump is short-cycling because of a hydraulic leak or a failed accumulator precharge, that repair is $500 to $1,500 and skipping it means the new relay will fail again. Where the relay is integrated into a sealed module, $200 to $600 fitted; where it is a module-internal driver, the repair becomes a control unit with programming.
Estimate your repair
Run the numbers for your vehicle
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 intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.