P0A7B means the Hybrid/EV Control ECU has detected that **Motor Electronics Coolant Pump “B” is stuck OFF**. Many hybrid and electric vehicles use multiple coolant pumps—Pump “A” typically cools the inverter and Motor A, while Pump “B” supports additional power electronics or the secondary drive motor. If Pump “B” fails to activate when commanded, coolant flow stops through its assigned circuit. This can quickly lead to inverter overheating, weak performance, or hybrid system shutdown. This guide explains exactly what P0A7B means, the causes, symptoms, diagnosis steps, and the correct fixes.
What Does P0A7B Mean?
P0A7B sets when the ECU commands Coolant Pump “B” to operate but detects **no current draw or no coolant flow**, indicating that the pump is not turning on. The ECU monitors pump feedback via voltage, current signals, and temperature changes in the electronics it cools. If the pump does not activate when expected—especially during high load or inverter warm-up—the ECU classifies it as “Stuck Off.”
Since Pump “B” helps maintain stable temperatures in secondary electronics, a stuck-off failure can cause rapid overheating, reduced hybrid performance, and possible inverter shutdown.
Quick Reference
- OBD-II Family: P-Code (Hybrid Cooling System)
- Scope: Generic
- System: Motor Electronics Coolant Pump “B”
- Difficulty Level: Moderate
- Estimated Repair Cost: €120–€450
- Last Updated: 2025-11-24
Real-World Example / Field Notes
We’ve seen P0A7B on Toyota Prius Plug-In, Lexus hybrids with dual cooling loops, Hyundai/Kia hybrids, and some Ford and GM EV systems. One Toyota Plug-In repeatedly overheated its secondary inverter because Pump “B” seized internally after 160,000 km—yet the primary pump still operated normally, making the diagnosis confusing. A Kia Niro Plug-In Hybrid set P0A7B after a coolant service where air trapped in the secondary circuit prevented Pump “B” from priming. A Ford C-Max Hybrid had a corroded ground wire causing zero current draw, falsely presenting as a failed pump.
Symptoms of P0A7B
- Secondary inverter overheating: Temperature rises quickly under load.
- Hybrid/EV warning lights: “Check Hybrid System,” “EV System Fault,” etc.
- Loss of EV mode (plug-ins): System avoids stressing electronics.
- Limp mode: Vehicle restricts performance to reduce heat.
- Cooling pump silent at all times: No buzzing or fluid movement.
- Weak regenerative braking: System limits regen to reduce heat generation.
- Propulsion shutdown (severe): READY mode may be blocked if temperatures exceed limits.
Common Causes of P0A7B
Most Common Causes
- Failed Coolant Pump “B” (internal motor or electronics failure).
- Open circuit in pump power or ground wiring.
- Air pockets in the secondary cooling loop.
- Low coolant level reducing flow and pump priming.
- Seized pump impeller due to wear or contamination.
Less Common Causes
- Faulty pump control module or relay (vehicle dependent).
- Incorrect pump installation after service.
- Damaged hoses or internal blockages.
- Hybrid ECU output driver failure (rare).
Diagnosis: Step-by-Step Guide
You must determine whether Pump “B” is truly stuck OFF or if the ECU is losing the power/ground signal needed to activate it.
Tools You’ll Need: OBD-II scan tool, multimeter, coolant funnel/bleeder, infrared thermometer, wiring diagrams, and insulated gloves.
- Command Pump “B” ON with a scan tool. Listen for operation and check for vibration at the pump body.
- Inspect coolant reservoir (if shared). Lack of turbulence indicates no flow.
- Check voltage at the pump connector. With the pump ON command, you should see system voltage.
- Test the pump ground. Corroded or floating grounds are common causes.
- Check pump resistance. Infinite resistance typically indicates internal pump failure.
- Inspect for air pockets. Bleed the secondary cooling system thoroughly.
- Verify coolant level and condition. Low coolant prevents pump priming.
- Check wiring for opens or breaks. Pay attention to sections near heat sources.
- Test pump with direct power (bench test). If it doesn’t spin, the pump is faulty.
- Review freeze-frame data. This reveals load/temperature conditions that triggered the fault.
Pro Tip: If Pump “B” measures correct voltage but still won’t spin, check current draw with an ammeter—zero current with correct voltage confirms seized impeller or internal motor burnout.
Possible Fixes & Repair Costs
- Replace Coolant Pump “B”: €150–€350 depending on model.
- Repair or replace corroded wiring/grounds: €40–€150.
- Bleed the secondary cooling loop: €40–€120.
- Replace damaged hoses or remove blockages: €30–€120.
- Replace pump fuse or relay (if equipped): €10–€40.
- Replace ECU driver module (rare): €250–€500.
Confirm power, ground, and coolant flow before replacing expensive components—most P0A7B cases are caused by a failed pump or bad wiring, not an ECU fault.
Can I Still Drive With P0A7B?
Driving with P0A7B is risky. If Pump “B” is stuck OFF, secondary electronics can overheat within minutes. You may lose EV mode, regenerative braking, or even propulsion entirely. If warning lights appear or the inverter temperature rises quickly, stop driving to avoid severe power electronics damage.
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
- P0A7B indicates Coolant Pump “B” is not turning on when commanded.
- Most causes involve pump failure, air pockets, or wiring issues.
- Driving with this code risks overheating and hybrid system shutdown.
- Verify power and ground before replacing the pump.
FAQ
What causes P0A7B most often?
Most failures come from a seized or electrically dead Pump “B,” or wiring faults preventing it from receiving power.
Can I drive with P0A7B?
Not safely. Overheating can occur quickly when Pump “B” is not running, especially under load or in warm weather.
How do I fix P0A7B?
Check pump power and ground, bleed the cooling loop, verify coolant level, inspect wiring, and replace the pump if it fails electrical tests.
Can air in the system trigger P0A7B?
Yes. Air pockets can prevent Pump “B” from circulating coolant, making the ECU think the pump is stuck OFF.
Does P0A7B mean the inverter is failing?
Usually not—the pump or wiring is the root cause. However, running the system without coolant flow can eventually damage the inverter.