fault code guideHybrid & EV
P0A78: Drive Motor 'A' Inverter Performance (Hybrid/EV)
The car's computer has detected an electrical fault inside the inverter that powers the main electric motor. You will likely lose electric power immediately or enter limp mode with little acceleration.
How it shows up: A master warning triangle, hybrid system fault light, or engine management light will appear on the dash, often accompanied by a 'Check Hybrid System' message. The car will usually lose drive completely or drop into a low-power limp mode.
Updated 9 November 2026
Can I keep driving?
Stop driving
Continuing to drive risks sudden total loss of propulsion in traffic and severe electrical damage to expensive hybrid components.
What to do right now
- 1Safely guide the car off the road or onto the hard shoulder before propulsion fails completely.
- 2Switch off the vehicle and do not attempt to clear the fault to continue your journey.
- 3Do not open or touch any high-voltage components or anything wrapped in bright orange cabling.
- 4Call roadside assistance to transport the vehicle to an EV or hybrid specialist.
Likely causes
Most likely first.
- Likely
Internal inverter assembly failure
Power transistors or circuit boards inside the inverter fail due to age, thermal stress, or internal electrical breakdown.
- Possible
Inverter cooling system failure
A failed dedicated electric inverter water pump or air lock causes the inverter to overheat rapidly, triggering this protection code.
- Possible
Drive motor stator or cable fault
Degraded insulation or a short circuit in the electric drive motor windings causes extreme current draw that trips the inverter.
- Rare
Faulty 12-volt auxiliary battery
A collapsing 12-volt battery causes low control voltage, confusing the computer that monitors the hybrid inverter.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Check inverter coolant reservoir level
Open the bonnet once the car is fully cooled down and locate the separate hybrid inverter coolant tank. Ensure the coolant level sits between the minimum and maximum lines without opening any pressurised caps.
- 2
Inspect the front radiator grille for blockages
Check the lower front bumper and radiator fins for road debris, plastic bags, or heavy leaf build-up that could block cooling airflow to the hybrid radiator.
- 3
Check 12-volt battery resting voltage
With the car completely powered off, measure the auxiliary 12-volt battery with a digital multimeter. A healthy battery should read around 12.6 volts; low voltage can cause hybrid control modules to log false inverter codes.
- 4
Leave orange high-voltage cables untouched
Inspect visual areas only from a distance; high-voltage parts marked with bright orange conduits carry lethal current and are strictly for trained technicians only.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The total cost depends mainly on whether the failure was caused by a separate cooling pump or requires a complete replacement inverter unit.
Hybrid inverter water pump replacement
- Do it yourself
- Garage job
- At a garage
- €320–650£280–580 in the UK
Inverter assembly replacement (reconditioned)
- Do it yourself
- Garage job
- At a garage
- €1,100–2,200£950–1,900 in the UK
New genuine inverter and converter assembly
- Do it yourself
- Garage job
- At a garage
- €2,800–5,200£2,400–4,500 in the UK
When to go to a garage
- The car displays a hybrid system warning and loses drive or drops into limp mode.
- The inverter coolant tank is empty or bubbling rapidly when the ignition is switched on.
- You suspect an electrical short or internal fault within the high-voltage drivetrain.
- The 12-volt battery is in good condition but the fault code returns immediately after clearing.
Questions drivers ask
Can I drive my car with code P0A78 present?
No, you should not drive the car. The inverter controls the flow of electricity to the wheels, and an active failure can leave you without power instantly on a live road. Continued driving also risks permanent, costly damage to the electric motor and high-voltage battery.
Can I repair or replace the inverter myself at home?
No, this is strictly a job for certified high-voltage technicians. Hybrid and electric vehicle inverters operate at hundreds of volts, which carries a severe risk of fatal electric shock. Specialist insulated tools, personal protective gear, and strict de-energising procedures are required.
Will code P0A78 cause an MOT or NCT failure?
On the UK MOT, an illuminated engine management light on a hybrid car is an automatic failure. On the Irish NCT, dashboard hybrid fault codes and engine lights give an Advisory Pass unless the issue prevents the car from completing the test safely or causes an emissions test abort. However, a car in limp mode cannot complete either test.
Are there recalls or warranty extensions for inverter faults?
Several hybrid manufacturers have issued recalls and extended warranty programmes covering inverter module failures and defective cooling pumps. Before paying for an expensive replacement, contact a franchised dealer with your car's registration and chassis number to check for outstanding campaigns.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
The warning light you may see
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.