P0A79 – Drive Motor “B” Inverter Voltage

P0A79 means the Hybrid/EV Control ECU has detected an **abnormal voltage condition inside the Drive Motor “B” inverter**. Drive Motor “B” (often MG1 or the secondary motor-generator depending on the vehicle) relies on stable inverter voltage for propulsion assistance, regenerative braking, and engine starting in many hybrid systems. When inverter voltage is too high, too low, or unstable, the ECU sets P0A79 to protect the power electronics. You may notice reduced hybrid performance, regen loss, warning lights, or limp mode. This guide explains P0A79 clearly—what it means, what causes it, and how to diagnose and repair it safely.

What Does P0A79 Mean?

P0A79 sets when the ECU detects that the inverter supplying Drive Motor “B” is operating outside its expected voltage range. The inverter regulates DC battery voltage into three-phase AC using IGBTs or MOSFETs, while a DC link capacitor stabilizes the internal DC bus. The ECU monitors voltage stability, switching frequency, and ripple voltage.

If Drive Motor “B” inverter voltage becomes unstable—because of overheating, weak capacitors, wiring corrosion, battery voltage fluctuations, or internal inverter failures—the hybrid system may reduce torque or disable EV mode to protect itself. Since Motor “B” often handles engine cranking or regen, voltage issues can noticeably affect drivability.

Quick Reference

• OBD-II Family: P-Code (Hybrid/EV Power Electronics)
• Scope: Generic
• System: Drive Motor “B” Inverter Circuit
• Difficulty Level: High
• Estimated Repair Cost: €500–€2,400+
• Last Updated: 2025-11-24

Real-World Example / Field Notes

We’ve seen P0A79 on Toyota Prius/Camry/RAV4 hybrids, Lexus CT/ES/RX, Hyundai/Kia hybrids, and certain GM hybrid models. One Prius exhibited P0A79 during regenerative braking—voltage spiked due to a degraded DC link capacitor inside the MG1 portion of the inverter. A Hyundai Ioniq Hybrid triggered the code because a corroded phase cable created voltage imbalance under load. Meanwhile, a Lexus RX450h repeatedly set the code only on hot days; the inverter coolant pump output was weak, causing voltage instability inside the MG1 inverter section. Voltage-related faults often show up only under specific conditions, making freeze-frame data critical.

Symptoms of P0A79

• Reduced hybrid power: Torque output is limited to protect the inverter.
• Weak regenerative braking: Regen may drop out under load.
• EV mode disabled: System avoids stressing Motor “B”.
• Hybrid warning lights: “Check Hybrid System” or similar alerts.
• Harsh transitions: Jerkiness during engine start or mode switches.
• Inverter overheating: Temp rises faster than normal.
• Limp mode activation: Power output severely reduced.

Common Causes of P0A79

Most Common Causes

• Failing internal inverter components (IGBT/MOSFET damage).
• Weak or degraded DC link capacitor causing high ripple voltage.
• Corroded or loose phase cable to Drive Motor “B”.
• Inverter overheating due to weak coolant pump or low coolant.
• Unstable HV battery voltage under load.

Less Common Causes

• Faulty inverter voltage sensor or logic board.
• Internal coolant contamination inside the inverter (rare).
• Damaged high-voltage connectors or terminals.
• Incorrect replacement inverter or improper installation.

Diagnosis: Step-by-Step Guide

Your goal is to determine whether the voltage instability comes from overheating, wiring issues, battery fluctuation, or internal inverter failure.

Tools You’ll Need: EV-capable scan tool, multimeter, oscilloscope (advanced), infrared thermometer, coolant bleeding tools, wiring diagrams, and HV-rated gloves.

1. Check inverter temperature. Overheating is one of the top triggers for voltage instability.
2. Inspect coolant level and pump flow. Look for reservoir turbulence and steady pump sound.
3. Review freeze-frame data. Determine if the fault occurred during regen, heavy load, or high inverter temperature.
4. Monitor DC link voltage. Look for spikes, drops, or rapid fluctuations.
5. Check HV battery block voltages. Weak modules can destabilize inverter voltage under acceleration.
6. Inspect MG1/Motor “B” phase cables. Look for heat discoloration, corrosion, or loose connections.
7. Check for temperature-related failures. Heat-soaked inverters often show voltage instability first.
8. Use an oscilloscope to measure voltage ripple (advanced). Excess ripple indicates bad capacitors.
9. Check inverter internal coolant leaks. Rare but catastrophic if coolant reaches electronics.
10. Test ground circuits and power distribution. Poor grounding can distort voltage readings.

Pro Tip: Compare inverter voltage behavior in three conditions—regen, acceleration, and steady cruise. If voltage spikes during regen but not acceleration, suspect capacitor degradation in the Motor “B” inverter circuit.

Possible Fixes & Repair Costs

• Replace inverter coolant pump: €150–€400.
• Repair high-voltage or phase cable connections: €40–€150.
• Replace DC link capacitor or inverter control board: €600–€1,800.
• Replace complete inverter assembly: €1,000–€2,400+.
• HV battery module repair or reconditioning: €300–€1,200.
• Install updated or correct inverter replacement: €700–€1,800.

Always confirm cooling system performance, voltage ripple, and cable condition before replacing expensive inverter hardware—many P0A79 issues start with overheating or loose connections.

Can I Still Drive With P0A79?

Driving with P0A79 is unsafe. Voltage instability can cause sudden loss of power, shutdown of regenerative braking, or inverter overheating. Limp mode may activate without warning. If the vehicle feels weak, overheats, or shows repeated hybrid warnings, stop driving to prevent major inverter or battery damage.

Last updated: January 22, 2026