Drivers who encounter the P2799 code often notice the transmission behaving oddly just after the check‑engine light illuminates. The most common first impression is a sudden shift that feels harsher than usual, followed by a brief delay before the next gear engages. In many cases a transmission‑temperature warning flashes on the dash, and the vehicle may enter limp‑mode, limiting gear selection to protect the drivetrain. Because the code specifically flags an “Electric/Auxiliary Transmission Fluid Pump A Control Circuit High,” the underlying issue is electrical rather than mechanical. Early identification of the over‑voltage condition prevents permanent damage to the pump driver or the TCM and avoids costly transmission repairs.
The auxiliary pump is driven by a low‑voltage motor whose controller expects a steady 5 V or 12 V signal. A fault that supplies higher voltage—often from a failing voltage regulator or a shorted power‑supply line—triggers the P2799 condition. The pump may stay off or spin at an incorrect speed, leading to the shift symptoms described above.
Corroded pins, cracked insulation, or loose connector clips can create intermittent shorts or ground paths. When the wiring spikes, the control module registers a “high” condition and logs P2799. Heat‑induced expansion of a harness bundle is a frequent source of these spikes.
The driver module that switches the pump motor on and off contains power transistors that can degrade over time. A shorted transistor can feed excess voltage back into the control line, producing the high‑circuit reading even if the TCM is otherwise healthy.
The TCM generates the command signal for the pump. Internal circuitry damage—often from moisture ingress or previous electrical surges—can cause the TCM to output a voltage that exceeds the specification, resulting in the same high‑circuit condition.
– Connect a professional scan tool (e.g., Bosch, Snap‑On) and read all pending and stored codes. Confirm that P2799 appears alongside any related transmission or TCM codes.
– Locate the auxiliary pump A harness near the transmission oil pan. Check connectors for corrosion, bent pins, or damaged sleeves. Repair or replace any compromised sections before proceeding.
– With the ignition on and the pump commanded (typically during a shift), measure the control line voltage at the pump connector. A reading above the manufacturer’s specification (often > 13 V for a 12 V system) confirms the “high” condition.
– Disconnect the pump motor and apply a known good voltage from a bench power supply. If the motor runs smoothly, the driver is likely at fault. Conversely, if the motor does not respond, the pump itself may be defective.
– Using the scan tool, command the pump on while monitoring the TCM’s output signal with a multimeter or oscilloscope. Consistently high voltage indicates a TCM fault; intermittent spikes point to wiring.
– Wiring: Replace corroded pins, reseat connectors, and apply dielectric grease to prevent future moisture intrusion.
– Pump Driver: Replace the driver module; cost typically $200‑$400 for parts plus $150‑$250 labor.
– TCM: If the TCM is the source, replacement is usually $600‑$900 for the unit plus $200‑$300 labor. Reprogramming to the vehicle’s VIN is required after installation.
– Clear all codes, perform a functional test by cycling through all gears, and verify that the transmission‑temperature warning does not reappear. Re‑scan to ensure no new codes are set.
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.