P0A7C means the Hybrid/EV Control ECU has detected that **Motor Electronics Coolant Pump “B” is stuck ON**, meaning it continues to run even when the ECU commands it OFF. Pump “B” supports cooling for secondary inverters, power electronics, or the second drive motor depending on the vehicle. A pump stuck ON may seem harmless, but it often indicates an electrical fault such as a shorted relay, internal pump short, or a stuck ECU driver transistor. This guide explains what P0A7C means, its causes, symptoms, diagnosis steps, and the correct repairs.
What Does P0A7C Mean?
P0A7C is stored when the ECU deactivates Coolant Pump “B” but still detects electrical activity—usually through continued current draw or unexpected voltage feedback. The inverter cooling system is tightly regulated, and Pump “B” should only run under specific temperature and load conditions. If it remains powered when commanded OFF, the ECU identifies a control failure.
Although the pump still functioning may not cause immediate overheating, a stuck-on pump can overheat itself, drain the 12-volt battery, disrupt cooling strategy, or signal wiring damage that could affect other components.
Quick Reference
- OBD-II Family: P-Code (Hybrid Cooling System)
- Scope: Generic
- System: Motor Electronics Coolant Pump “B” Control
- Difficulty Level: Moderate
- Estimated Repair Cost: €80–€350
- Last Updated: 2025-11-24
Real-World Example / Field Notes
P0A7C appears on Toyota Prius Plug-In, Lexus hybrid systems with dual-loop cooling, Hyundai/Kia hybrids, and some Ford and GM EV platforms. One Prius Plug-In had Pump “B” running continuously—even with the car off—because an aftermarket replacement pump had an internal short tying the control and power circuits together. A Kia Niro Plug-In Hybrid exhibited this code due to a stuck relay that welded closed from heat cycles. A Ford Fusion Hybrid showed P0A7C after water intrusion corroded the pump connector, causing stray voltage that mimicked a constant ON signal.
Symptoms of P0A7C
- Pump runs continuously: Audible buzzing even when the vehicle is not in READY mode.
- Unexpected 12V battery drain: Pump staying ON after shutdown pulls excessive current.
- Hybrid/EV warning messages: “Check Hybrid System” or cooling system errors.
- Cooling imbalance: Inverter temperatures may fluctuate improperly.
- Premature pump wear: Constant operation shortens pump lifespan.
- Overheating pump motor: Pump may run hot from continuous use.
Common Causes of P0A7C
Most Common Causes
- Stuck or welded coolant pump relay (vehicle dependent).
- Shorted internal electronics inside Pump “B”.
- Short-to-power in the pump control or power circuit.
- ECU driver transistor stuck in ON state.
- Water intrusion or corrosion causing unintended voltage feedback.
Less Common Causes
- Incorrect aftermarket pump with miswired internal circuitry.
- Damaged wiring insulation allowing backfeed from adjacent circuits.
- Poor ground connection creating false ON signals.
- Cooling system modifications or incorrect repairs.
Diagnosis: Step-by-Step Guide
Your objective is to identify whether the pump is being held ON mechanically, electrically, or through unintended feedback to the ECU.
Tools You’ll Need: OBD-II scan tool, multimeter, test light, wiring diagrams, infrared thermometer, and insulated gloves.
- Listen for pump operation after shutdown. Pump “B” should turn off shortly after READY mode ends.
- Use a scan tool to command Pump “B” ON and OFF. Note any difference in pump sound or coolant flow.
- Measure voltage at the pump connector. If voltage remains present when OFF is commanded, trace the source.
- Check the pump relay (if applicable). Relays that weld shut will power the pump constantly.
- Inspect for short-to-power in wiring. Pay attention to areas near hot components or sharp edges.
- Unplug the pump. If the ECU still reports ON feedback, a wiring or ECU driver issue is likely.
- Check pump ground quality. Poor grounds can create partial voltage flow.
- Look for water intrusion. Corroded connectors frequently cause unintended continuity.
- Voltage-drop test the control circuit. A stuck ON condition may be due to small backfeed voltages.
- Review freeze-frame data. Helps determine when the system first detected abnormal pump activity.
Pro Tip: If pump voltage is present with the OFF command and the relay tests fine, disconnect the ECU control wire to isolate whether the ECU is inadvertently providing continuity or if there is a harness short feeding power into the circuit.
Possible Fixes & Repair Costs
- Replace Coolant Pump “B”: €120–€300 if the pump is internally shorted.
- Replace stuck relay or fuse (if applicable): €10–€40.
- Repair short-to-power wiring faults: €40–€150.
- Clean or repair corroded connectors: €20–€80.
- Replace ECU driver stage (rare): €250–€500.
- Correct faulty aftermarket installation: €50–€120.
Always test voltage and wiring paths before replacing the pump—many P0A7C cases are caused by an external short or stuck relay rather than a faulty pump.
Can I Still Drive With P0A7C?
Driving is usually possible, but not recommended. A pump stuck ON can drain the 12-volt battery after shutdown, overheat the pump motor, or signal a wiring issue that may affect other circuits. If hybrid system warnings appear or if the pump continues running after shutdown, stop driving until the issue is diagnosed.
Related Codes
- P0A6C – Hybrid Battery Voltage System Isolation Sensor Range/Performance
- P0A9F – Hybrid Battery Temperature Sensor Too Hot
- P0A9E – Hybrid Battery Temperature Sensor Too Cold
- P0A9D – Hybrid Battery Temperature Sensor Range/Performance
- P0A9C – Motor Electronics Coolant Temperature Sensor Circuit High
- P0A9B – Motor Electronics Coolant Temperature Sensor Circuit Low
- P0A9A – Motor Electronics Coolant Temperature Sensor Circuit Range/Performance
- P0A99 – Motor Electronics Coolant Temperature Sensor Circuit
- P0A98 – Hybrid Battery Pack Cooling Fan 2 Performance
- P0A97 – Hybrid Battery Pack Cooling Fan 2 Control Circuit
Key Takeaways
- P0A7C means Coolant Pump “B” remains ON even when the ECU commands it OFF.
- Common causes include a stuck relay, shorted pump, or wiring backfeed.
- Driving can lead to pump overheating or 12V battery drain.
- Confirm wiring and control signals before replacing the pump.
FAQ
What causes P0A7C most often?
A stuck relay or shorted internal pump electronics is the most common cause.
Is it safe to drive with P0A7C?
Driving is possible but unsafe. The pump may drain the 12V battery or suffer premature failure from running continuously.
How do I diagnose P0A7C?
Use a scan tool to command the pump, check voltage at the connector, inspect wiring for shorts, and test the relay and ground circuits.
Can aftermarket pumps cause this?
Yes. Some low-quality replacements have incorrect internal wiring that causes constant pump operation.
Does P0A7C mean the ECU is bad?
Rarely. ECU driver faults can cause this, but most cases involve relays, shorts, or pump internal failure.