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P2798 — Electricauxiliary Transmission Fluid Pump Control Circuit Low

Manufacturer-specificPowertrain (engine & transmission)

The code signals that the PCM has detected insufficient voltage or a fault in the circuit that drives the electric auxiliary transmission fluid pump. This pump supplies pressurized fluid for smooth shifting and torque converter operation.

Symptoms

What causes it — most likely first

1. Faulty auxiliary transmission pump

The pump motor can fail internally, losing the ability to draw power and causing low circuit voltage.

2. Wiring or connector problem

Corroded, damaged, or loose wiring or connector can restrict voltage to the pump, triggering the low circuit reading.

3. Blown fuse

A fuse protecting the pump circuit may have blown, cutting power to the pump and causing a low voltage condition.

4. Faulty relay

If the relay that supplies power to the pump fails, the pump will not receive voltage despite a healthy circuit.

5. PCM output driver issue

The PCM's control output for the pump may be defective, but this is rare and is considered only after other components are verified.

How it's diagnosed

  1. Inspect the pump, wiring, and connectors for visible damage or corrosion.
  2. Check the pump circuit fuse and replace it if blown.
  3. Test the relay for proper operation and swap it if suspect.
  4. Measure voltage at the pump connector while the transmission is commanded to engage the pump.
  5. If voltage is present, test the pump motor for resistance and functional operation; replace the pump if faulty.
  6. If all hardware checks pass, evaluate the PCM output circuit for faults before considering PCM replacement.

Code facts & related codes

SystemPowertrain (engine & transmission)
StandardizationManufacturer-specific (P1/B1/C1/U1)

Frequently asked

Can I fix P2798 by simply replacing the PCM?

Replacing the PCM is rarely necessary; the issue is usually a failed pump, wiring problem, blown fuse, or bad relay, which are far more common causes.

Will the transmission shift properly after the pump is repaired?

Yes, once the pump can maintain proper pressure, shifting should return to normal; however, a lingering fault may require clearing the code and a test drive.

Is the pump covered under warranty?

Warranty coverage depends on the vehicle’s age, mileage, and manufacturer policies; the diagnostic process will determine if the repair qualifies.

Do I need to drain the transmission fluid to replace the pump?

In most designs the pump is external to the transmission housing, so fluid does not need to be drained, but some models may require partial drainage for access.