P0A77 means the Hybrid/EV Control ECU has detected a **performance problem with Motor Electronics Coolant Pump “A”**. This electric pump circulates coolant through the inverter and power electronics, keeping temperatures stable under load. When the ECU sees reduced flow, weak pump speed, or inconsistent electrical behavior, it flags this code. You may notice hybrid warning lights, sluggish acceleration, reduced EV mode, or inverter temperature rising faster than normal. Because the inverter is very sensitive to cooling issues, this fault should not be ignored. This guide explains the meaning, causes, diagnosis, and fixes for P0A77 clearly.
What Does P0A77 Mean?
P0A77 sets when the ECU commands Motor Electronics Coolant Pump “A” to operate but detects **insufficient coolant flow or pump output not matching expected performance**. The ECU monitors pump current draw, coolant temperature changes over time, and inverter cooling behavior. If coolant flow is too low, inconsistent, or delayed, the system classifies this as a performance issue.
Unlike a stuck ON/OFF condition, P0A77 indicates the pump is technically running but **not moving enough coolant**. This can cause the inverter to run hotter than intended, especially during high-load driving or regenerative braking.
Quick Reference
- OBD-II Family: P-Code (Hybrid Cooling System)
- Scope: Generic
- System: Inverter / Motor Electronics Cooling
- Difficulty Level: Moderate
- Estimated Repair Cost: €120–€450
- Last Updated: 2025-11-24
Real-World Example / Field Notes
We’ve seen P0A77 across Toyota Prius/Camry hybrids, Honda Insight, Kia/Hyundai hybrids, and GM Voltec systems. One Prius customer had strong pump activation noise but very little coolant movement—internal bearing wear caused the impeller to drag and slow down under heat. A Hyundai Ioniq Hybrid had a partially clogged inverter cooling passage that reduced circulation only at high temperatures, causing intermittent P0A77. Another case involved an aftermarket replacement pump with weaker output than OEM, triggering the code during long climbs. Performance faults often appear only under heavy load, making them trickier than simple ON/OFF failures.
Symptoms of P0A77
- Reduced coolant flow: Minimal turbulence in the inverter reservoir.
- Hybrid system warning lights: “Check Hybrid System” or similar alerts.
- Loss of EV mode: The ECU avoids additional inverter load.
- Sluggish performance: Reduced power during acceleration.
- Cooling fans running frequently: System compensates for weak coolant flow.
- Inverter running hotter than normal: Temperature climbs unusually quickly.
- Limp mode under high load: Hybrid protection strategy activates.
Common Causes of P0A77
Most Common Causes
- Worn or weak Motor Electronics Coolant Pump “A”.
- Clogged cooling passages reducing flow.
- Air trapped in the inverter cooling system.
- Low coolant level causing cavitation.
- Failing pump bearing or partially seized impeller.
Less Common Causes
- Incorrect aftermarket pump with insufficient output.
- Electrical supply issues causing reduced pump speed.
- Intermittent ground or voltage drop.
- Contaminated coolant reducing heat transfer.
Diagnosis: Step-by-Step Guide
Your goal is to confirm whether the pump is weak, flow is restricted, or there’s a system-related electrical issue.
Tools You’ll Need: OBD-II scan tool, multimeter, coolant funnel/bleeder, infrared thermometer, wiring diagrams, and insulated gloves.
- Command the pump ON via scan tool. Listen for operation and observe RPM or duty-cycle data if available.
- Check coolant reservoir movement. Strong turbulence indicates good pump flow; weak movement suggests low performance.
- Inspect coolant level. Low level leads to cavitation and reduced flow.
- Bleed the cooling system. Air pockets commonly cause P0A77 after coolant service.
- Check pump voltage under load. Low voltage causes weak output—trace wiring if voltage drops when the pump activates.
- Measure inverter temperature rise. Quick overheating confirms low coolant flow.
- Inspect coolant quality. Sludge or debris restricts circulation.
- Check hoses for internal collapse. Older hoses may kink under vacuum.
- Disconnect the pump and test resistance. High resistance indicates internal wear.
- Review freeze-frame data. Look for high-load, long climbs, or high-temperature conditions that triggered the fault.
Pro Tip: On Toyota hybrids, evaluate Mode $06 pump performance fields—look for commanded duty cycle versus actual temperature drop. A weak pump often shows high duty cycle but minimal cooling efficiency.
Possible Fixes & Repair Costs
- Replace Motor Electronics Coolant Pump “A”: €150–€350 for most hybrids.
- Bleed and refill inverter coolant: €40–€120.
- Replace collapsed or clogged hoses: €30–€150.
- Clean or flush inverter cooling passages: €80–€200.
- Repair voltage drop or ground issues: €40–€150.
- Replace contaminated coolant: €80–€120.
Confirm flow and pump voltage before replacing the pump—performance issues often come from air pockets or coolant flow restrictions rather than outright pump failure.
Can I Still Drive With P0A77?
Driving with P0A77 is risky because the inverter may overheat under load. The hybrid system may reduce power or enter limp mode unexpectedly. If you see rising inverter temperature, reduced EV mode, or frequent warning messages, stop driving to prevent overheating 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
- P0A77 indicates low or inconsistent performance from coolant pump “A”.
- Common causes include a weak pump, air pockets, or restricted flow.
- Driving with this fault risks inverter overheating and limp mode.
- Always check coolant flow before replacing costly components.
FAQ
What causes P0A77 most often?
A weak or partially failing coolant pump is the most common cause, followed by air pockets or restricted coolant passages.
Is it safe to drive with P0A77?
Not recommended. The inverter may overheat under load, causing sudden power loss or hybrid shutdown.
How do I fix P0A77?
Inspect coolant level, bleed the system, verify pump voltage, check flow strength, and replace the pump if it cannot produce normal circulation.
Can contaminated coolant cause P0A77?
Yes. Sludged or degraded coolant reduces heat transfer and may block passages, lowering pump performance.
Does P0A77 mean the pump is failing?
Often yes, but not always—air pockets, low coolant, electrical issues, or partial blockages can also cause performance faults.