P0A78 on Toyota: How to Diagnose & Fix
Drive Motor A Inverter Over-Temperature
2 Toyota models · 2 documented issues · 2 critical · $200-$4,500 repair
P0A78 on Toyota by model: symptoms, diagnosis and fix
The Prius C has a separate cooling circuit for the power electronics (inverter/converter). The electric pump that circulates coolant through this circuit can fail, causing the inverter to overheat. When the inverter overheats, the hybrid system enters a protective shutdown mode, displaying "Hybrid System Malfunction" and severely limiting power output. The pump failure is often caused by coolant contamination or bearing wear.
Common Symptoms
- Hybrid System Malfunction warning message
- Triangle warning indicator (master caution) illuminated
- Reduced power output / vehicle feels sluggish
- No EV-only mode available
- Inverter cooling fan running at high speed constantly
How to Fix
Replace the inverter coolant pump (Toyota G9040-52010). Flush the inverter cooling circuit with distilled water then refill with Toyota Super Long Life Coolant (SLLC). Check the inverter coolant reservoir for contamination — brown or rusty fluid indicates the pump has been shedding bearing material. The pump is accessible from the engine bay and is a 1-hour repair.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Toyota G9040-52010 inverter water pump assembly with motor
replace a confirmed failed inverter coolant pump and restore/bleed the inverter cooling loop Fitment/availability: 2012-2018 Prius C 1.5L 1NZ-FXE inverter cooling circuit; source year 2019 requires VIN confirmation because the live catal…
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Owner tips & cautions
- TipChange the inverter coolant every 100,000 miles with Toyota SLLC — most owners forget this separate cooling circuit exists
- WarningDo not ignore the Hybrid System Malfunction warning — continued driving with an overheated inverter can destroy the power electronics ($3,000+ replacement)
Sources for this issue (3)
On the first- and second-generation Highlander Hybrid (2GR-FXE / 3MZ-FE hybrid), the Intelligent Power Module inside the hybrid inverter assembly carries control-board transistors whose operating temperature can exceed what the solder beneath them tolerates. The solder degrades, the transistor is heat-damaged, and the hybrid system throws multiple warning lamps. In most cases the vehicle drops into fail-safe/limp-home mode with sharply reduced power; in worse cases the power-supply-circuit fuse blows and the entire hybrid system shuts down while driving, stalling the vehicle. This is distinct from the inverter coolant pump failure and from HV traction-battery degradation — here it is the power electronics themselves, not the cooling loop or the pack. Toyota first recalled 2006-2007 cars (11V342000) and then broadened it to all 2006-2010 Highlander Hybrids built Feb 2005 through Jul 2010 under 13V396000 / Toyota campaign D0M.
Common Symptoms
- Multiple warning lights on at once — check engine, slip indicator, brake system, master warning
- Sudden loss of power / limp-home mode
- Hybrid system shuts down and vehicle stalls while driving
- Vehicle will not restart after shutdown
- Reduced acceleration, dashboard warning chime
How to Fix
Free recall remedy: dealers replace the Intelligent Power Module inside the inverter assembly at no cost under NHTSA 13V396000 (Toyota D0M) — check the VIN at nhtsa.gov/recalls, because many of these cars have changed hands several times and never had it done. If the VIN is not covered or the campaign has been performed and the fault returns, the repair is inverter assembly replacement (or specialist IPM board rework), which is the expensive path.
Sources for this issue (4)
P0A78 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0A78 mean on a Toyota?
P0A78 stands for "Drive Motor A Inverter Over-Temperature." P0A78 is a hybrid/electric-vehicle code indicating that drive motor 'A' inverter has overheated. The inverter converts the high-voltage battery's DC power into AC to drive the electric traction motor, and it has temperature sensors; when the inverter temperature exceeds safe limits, the control module sets this code and usually reduces power to protect the components. Symptoms include reduced power (limp mode), warning lights, or in severe cases the vehicle refusing to drive. It points to an inverter cooling or electronics problem in the high-voltage drive system rather than the gasoline engine. On Toyota it is documented across 2 models: Prius C, Highlander.
What are the most common causes of P0A78?
The most common causes of P0A78 are: Low coolant or air in the inverter cooling loop, Failed inverter/electronics coolant pump, Clogged or restricted inverter cooling passages/radiator, Faulty inverter temperature sensor, Failed or degrading inverter/power module, Cooling fan or thermostat fault in the inverter loop, Excessive load or related high-voltage system fault. The specific cause varies by vehicle.
How much does it cost to fix P0A78 on a Toyota?
Published repair estimates on Toyota range from $200 to $4,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0A78 on a different make?
View P0A78 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.