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DTC codes / U06xx · Network communication / U0694

U0694Lost Communication With Hybrid/EV Battery Pack Coolant Pump A

U0694 is a generic network communication code indicating that a control module on the vehicle's Controller Area Network (CAN) bus has lost contact with the hybrid or electric vehicle battery pack coolant pump, designated 'A'. The module that sets this code, typically the hybrid powertrain control module or battery energy control module, expects a continuous stream of status messages from the pump and flags a fault when those messages stop arriving within a calibrated timeout window.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaNetwork communication

Symptoms you may notice

  • Illumination of the hybrid system warning light or a generic check engine light, often with no other noticeable symptoms.
  • Reduced hybrid system power or a forced limp-home mode as the vehicle protects the battery pack from overheating.
  • Active cooling fan may run continuously or at maximum speed because the pump's speed feedback is missing.
  • In some vehicles, the engine may fail to start or the vehicle may refuse to enter 'Ready' mode if the pump is part of the battery thermal management enable logic.
  • Intermittent warning light that appears during cold starts or after the vehicle sits unused for several days.

Common causes, most likely first

  1. A failed coolant pump motor or internal pump electronics, which is the most common cause on high-mileage hybrids.
  2. Damaged, corroded, or disconnected electrical connector at the pump, often due to road debris or coolant intrusion.
  3. Open or shorted wiring in the pump's power, ground, or CAN communication circuit, frequently from rodent damage or chafing.
  4. A blown fuse or faulty relay in the pump's power supply circuit, especially if the pump shares a fuse with other cooling system components.
  5. Low or contaminated coolant that has caused the pump to seize or overheat, leading to internal failure.
  6. A software or calibration issue in the control module that monitors the pump, requiring a reprogramming update.
  7. A faulty control module that is not properly transmitting or receiving on the CAN bus, though this is less common than pump-side failures.

How to diagnose and fix it

  1. Perform a visual inspection of the coolant pump, its connector, and the wiring harness for signs of damage, corrosion, or coolant leakage.
  2. Check all fuses and relays that supply power to the coolant pump, and verify battery voltage and ground integrity at the pump connector.
  3. Use a scan tool to command the coolant pump on and monitor live data for pump speed and current draw to confirm whether the pump responds.
  4. Perform a voltage drop test on the pump's power and ground circuits while the pump is commanded on to rule out excessive resistance.
  5. Check for continuity and short-to-ground on the CAN communication wires between the pump and the control module, comparing to manufacturer specifications.
  6. If electrical checks pass, substitute a known-good pump or perform an actuator test to confirm pump operation before replacing parts.
  7. After repair, clear the code and verify communication is restored by running the vehicle through a complete drive cycle and monitoring for code return.

What Most Technicians Miss on U0694

  • A pump that spins freely by hand can still be electrically failed; the internal Hall-effect sensor or driver circuit often fails while the impeller remains mechanically intact.
  • Coolant pump communication codes frequently set as a result of low 12-volt battery voltage during cranking, so always check the battery state of charge before condemning the pump.
  • On many hybrids, the battery pack coolant pump is powered through a dedicated relay that can fail intermittently when hot, causing a code that only appears after a long drive.
  • A U0694 can be set by a control module that has lost its ground connection, which corrupts the entire CAN bus and makes the pump appear offline even when it is functional.
  • After replacing the pump, some vehicles require a relearn or initialization procedure through the scan tool to register the new pump's serial number with the control module.

Can I keep driving with U0694?

It is generally safe to drive a short distance to a repair shop, but avoid heavy acceleration, prolonged idling, or high ambient temperatures because the battery pack may overheat without coolant circulation. If the hybrid system warning light is accompanied by a power reduction message, stop driving and have the vehicle towed to prevent damage to the high-voltage battery.

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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