OBD-II trouble code
P0946: Hydraulic Pump Relay Circuit Range/Performance
The pump relay is not doing what it was told. Unlike the rest of this family, the usual fault here is a pump that will not switch OFF — which is why the first complaint is often a flat battery rather than a transmission problem.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $1,400
- DIY difficulty
- Intermediate DIY
Browse every code in P0900–P0999, or start from the full code library.
What does P0946 mean?
Every other code in this family describes a circuit that has gone missing. P0946 describes one that is present, connected, and behaving wrongly — and in a specific direction that makes it the odd member of the group.
The module commands the pump relay on and off and then checks whether the world agrees. It can watch the load side, the feedback pin, the supply voltage at the pump, or simply how the hydraulic pressure responds to a command. A range/performance verdict is stored when the commanded state and the observed state disagree in a way that is not a clean open and not a clean short. In practice that disagreement is most often in one direction: the relay is closed when it was told to open.
That matters because a relay switching a motor fails towards being stuck on. Every time the contacts break a motor circuit the collapsing field draws an arc across the gap, and every arc moves a little metal. After enough cycles the contact faces are pitted, roughened and, eventually, tacked together. A relay that dies from wear on a motor load is far more likely to weld shut than to fail open, which is exactly the opposite of the intuition most people bring to a relay fault.
The consequence is not a transmission symptom at all, which is why this code is so often misread. A hydraulic pump that never stops keeps running with the ignition off. The customer parks the car on Friday, comes back on Monday, and the battery is flat — and no one connects a dead battery to a gearbox. If a vehicle is presented for repeat battery drain and this code is in memory, the two facts are the same fact. A parasitic draw test will show a current far above the milliamps a sleeping car should pull, and it will not drop when the usual suspects are pulled, because the load is a motor being fed through contacts that are physically stuck together.
There is a second, quieter cost. The pump is designed for intermittent duty. It builds pressure into an accumulator, stops, and waits until pressure decays. Running it continuously means it never gets the off-time it was thermally designed around, so it runs hot, its brushes and bearings wear at many times the intended rate, and the fluid it is churning heats up too. A welded relay left in service for weeks does not just flatten batteries — it can take the pump with it, converting a cheap part into an expensive one purely through delay.
The honest exception is worth stating: a range/performance verdict can also be set by a relay whose contacts have gone high-resistance rather than stuck, so the pump runs but weakly and pressure builds too slowly. That version behaves like the low-side codes and is covered there. The distinguishing question is simple and can be answered in the driveway — with the key out and the car locked, does the pump keep running?
Common causes
- Relay contacts welded closed by arcing on the pump motor load
- Relay contacts pitted and high-resistance, so the pump runs weakly
- Control-side driver in the module stuck on, holding the coil energised
- Short to power on the relay control wire, energising the coil continuously
- Wrong relay fitted during previous work — undersized or wrong pin layout
- Feedback or monitor wire to the module open, so state cannot be confirmed
- Corroded relay socket producing intermittent contact behaviour
- Failed hydraulic pressure switch causing the module to misjudge pump response
- Aftermarket accessory wiring backfeeding the relay control circuit
- Relay damaged by prolonged operation at elevated under-bonnet temperature
Symptoms
- Battery flat after the car has stood for a few days
- Faint electric motor audible with the ignition off and the car locked
- Parasitic drain measured well above normal and not traced to a usual suspect
- Hydraulic pump running far more of the time than it used to
- Transmission fluid or actuator unit running unusually hot
- Pressure building slowly on the weak-contact version of this fault
- Check engine light with a transmission warning but no shifting complaint
- Repeat jump-starts with no charging system fault found
- Relay audibly buzzing or hot to the touch
- Pump noise that does not stop after the expected few seconds
Diagnostic steps
- 1.Before anything electrical, lock the car, wait for it to sleep, and listen at the pump — a motor still running with the key out is the whole diagnosis.
- 2.Perform a parasitic draw test and note whether the current falls when the pump relay is physically removed.
- 3.Check whether the vehicle has a history of unexplained battery drain, which is the most common presenting complaint for this code.
- 4.Command the relay off with a scan tool while measuring current in the pump feed — current still flowing means the contacts are welded.
- 5.Measure voltage on the relay coil control wire with the relay commanded off; battery voltage present indicates a short to power or a stuck driver.
- 6.Pull the relay and check the contact faces if it is a serviceable type — pitting and transferred metal are visible.
- 7.Compare the part number of the fitted relay against specification, since an undersized substitute welds quickly on a motor load.
- 8.If the pump runs but pressure builds slowly, measure voltage drop across the closed contacts under load rather than assuming the relay is fine.
- 9.Verify the feedback or monitor circuit back to the module, because an open there produces this verdict with a perfectly good relay.
- 10.After replacement, re-test parasitic draw with the car asleep to confirm the drain is genuinely gone.
Repair cost
$90 – $1,400
Diagnosis is $90 to $220, and a parasitic draw test may be billed on top at $80 to $150. A plug-in relay is $12 to $60 fitted in under half an hour, so many cars leave for $90 to $200. Add a battery if it has been deep-cycled repeatedly by the drain — $150 to $350. Where the relay lives inside a sealed fuse and relay module, budget $200 to $600 fitted. If the pump has been damaged by continuous running, the hydraulic power unit is the real bill at $700 to $1,400 or more. Control-wire or module-driver faults are quoted separately.
Estimate your repair
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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.