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DTC codes / P0Cxx · Hybrid propulsion / P0C29

P0C29Electric/Auxiliary Transmission Fluid Pump A Driver Circuit Performance

P0C29 indicates that the powertrain control module has detected a performance fault in the driver circuit for the electric or auxiliary transmission fluid pump, typically meaning the pump is not drawing the current or achieving the pressure the module expects during commanded operation. The circuit may be open, shorted, or the pump itself may be failing to respond correctly to the control signal.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaHybrid propulsion

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light
  • Often no symptoms besides the warning light, especially if the mechanical pump still provides adequate pressure
  • Harsh or delayed shifting, particularly when the engine is idling or during low-speed maneuvers
  • Transmission slipping or inability to engage a gear from a stop
  • Reduced fuel economy or altered shift feel as the transmission enters a limp-home mode
  • Overheating of the transmission fluid in prolonged stop-and-go driving if the auxiliary pump is completely inoperative

Common causes, most likely first

  1. Faulty electric auxiliary transmission fluid pump motor or internal pump wear
  2. Damaged, corroded, or disconnected wiring or connector at the pump or control module
  3. Blown fuse or failed relay supplying power to the auxiliary pump circuit
  4. Failed pump driver module or internal driver circuit within the transmission control module
  5. Low or contaminated transmission fluid causing pump cavitation or increased current draw
  6. Restricted transmission fluid filter or cooler line reducing pump efficiency and triggering a performance fault

How to diagnose and fix it

  1. Verify battery voltage and inspect the auxiliary pump fuse and relay, replacing any blown fuse or faulty relay first
  2. Perform a visual inspection of the pump connector and harness for corrosion, chafing, or loose pins, and repair as needed
  3. Use a scan tool to command the auxiliary pump on and off while monitoring live data for current draw and pump speed to confirm the fault
  4. Measure resistance and voltage drop across the pump motor circuit to check for excessive resistance or an open circuit
  5. Check transmission fluid level and condition, and inspect the filter for clogging, servicing the fluid and filter if contaminated
  6. If the circuit and fluid are good, test the pump motor directly with a known-good power source to confirm whether the pump itself has failed
  7. Replace the auxiliary transmission fluid pump or the pump driver module only after confirming the fault is internal to the component

When the pump circuit tests good but the code returns

  • A common misdiagnosis is replacing the auxiliary pump when the real fault is a corroded ground connection or a chafed wire hidden inside the harness loom, so always perform a voltage drop test under load.
  • The code can set intermittently due to a failing relay that works when cold but drops out when hot, so testing the relay with a load or swapping it with a known-good identical relay is a smart early step.
  • Low transmission fluid level from a slow leak can cause the auxiliary pump to cavitate and draw excessive current, setting P0C29 without any visible leak under the car.
  • Some vehicles use the auxiliary pump only during engine stop-start events, so the fault may not appear during a normal road test unless the scan tool actively commands the pump on.
  • A weak battery or charging system voltage below specification can cause the pump driver to log a performance fault even though the pump and wiring are healthy, so always check system voltage first.

Can I keep driving with P0C29?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy loads, trailer towing, or prolonged stop-and-go traffic because the auxiliary pump may not be cooling or maintaining pressure. If the transmission slips, overheats, or enters limp mode, stop driving and arrange a tow to prevent internal damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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