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DTC codes / P0Cxx · Hybrid propulsion / P0C23

P0C23Electric/Auxiliary Transmission Fluid Pump Control Module Circuit/Open

P0C23 indicates that the transmission control module (TCM) has detected an open circuit in the wiring or internal connection for the electric auxiliary transmission fluid pump control module. This pump is used in many modern automatic transmissions to maintain fluid pressure when the engine is stopped, such as during idle-stop events, and an open circuit means the TCM cannot command the pump to run.

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

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the dashboard
  • Often no symptoms besides the warning light, especially if the pump is only needed during auto-stop events
  • The engine may refuse to enter auto-stop mode or may restart abruptly
  • Harsh or delayed transmission shifts if the auxiliary pump is needed for pressure maintenance
  • Transmission may enter limp mode or fail to engage drive after an auto-stop
  • A stored code P0C23 is the primary clue, often without any drivability complaint

Common causes, most likely first

  1. Open or damaged wire in the power or ground circuit between the TCM and the auxiliary pump control module
  2. Corroded or disconnected electrical connector at the auxiliary pump or control module
  3. Failed auxiliary transmission fluid pump control module (internal open circuit)
  4. Blown fuse or open relay supplying power to the auxiliary pump circuit
  5. Faulty transmission control module (TCM) output driver for the pump circuit
  6. Physical damage to the pump wiring harness from road debris or heat

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related TCM or pump codes
  2. Inspect the auxiliary pump fuse and relay, and test for power and ground at the pump connector
  3. Visually inspect the wiring harness and connectors for chafing, corrosion, or loose pins
  4. Perform a voltage drop test on the power and ground circuits to confirm an open circuit
  5. Check the control module connector at the TCM for tightness and corrosion
  6. If wiring and connectors are good, test the auxiliary pump control module with a known-good unit or follow manufacturer diagnostics
  7. Repair or replace the faulty component (wire, connector, pump module, or TCM) and clear the code

What the open circuit really means

  • An open circuit does not always mean the pump itself has failed; a simple unplugged connector or broken wire is a common cause that is often overlooked.
  • The auxiliary pump may only run during engine auto-stop, so the driver may never notice a problem until the code is set and the stop-start feature is disabled.
  • Some vehicles use the auxiliary pump for cooling and lubrication during engine off, so ignoring the code can lead to transmission overheating in extended idle-stop conditions.
  • A faulty TCM can set this code even when the pump and wiring are intact, so always test the TCM output before replacing the pump.
  • After repairs, the TCM may require a relearn or adaptation reset to allow the auxiliary pump to operate normally again.

Can I keep driving with P0C23?

You can usually drive the vehicle with this code, but the automatic stop-start function may be disabled and the transmission may not maintain pressure during engine-off events. Avoid relying on auto-stop and have the circuit inspected soon to prevent potential transmission 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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