Check Hybrid / EV System Warning Light

Can I keep driving?

What to do right now

1. 1Move safely out of traffic and park in a safe location such as a car park or hard shoulder.
2. 2Switch off the car completely, wait three minutes for the computers to go to sleep, and switch it back on to see if the READY light appears.
3. 3If the warning stays on or the car refuses to enter READY mode, do not attempt to force it to drive; call for roadside assistance.
4. 4Never touch, move or open any bright orange cables or components under the bonnet or under the car.

Likely causes

Most likely first.

1. Likely

   Weak 12-volt auxiliary battery

   Hybrid and electric cars use an ordinary 12-volt battery to start the computer systems. If its voltage drops below normal resting levels, the control computers misread sensors and trigger false hybrid drive warnings.

2. Possible

   Inverter water pump or cooling failure

   The power inverter converts battery power for the electric motor and generates heavy heat. If its dedicated electric coolant pump stops or air locks form in its cooling loop, the inverter rapidly overheats.

3. Possible

   High-voltage traction battery degradation

   Individual cells or blocks within the main drive battery pack can lose capacity or drift out of balance. When the battery management computer detects voltage differences between cell blocks, it logs a system fault.

4. Rare

   High-voltage isolation or contactor fault

   The car continuously monitors for electrical leakage between high-voltage circuits and the metal chassis. If moisture enters a high-voltage connector or a main contactor switch sticks, the system locks out for safety.

What you can check yourself

Park somewhere safe and level, engine off, before you start.

1. 1

   Check the 12-volt battery terminals

   Open the bonnet or boot floor where your 12-volt auxiliary battery is housed. Check that both terminal clamps are clean, tight and free of white powdery corrosion without touching any orange cables nearby.

2. 2

   Check the inverter coolant expansion tank

   Locate the separate, smaller coolant tank dedicated to the hybrid or inverter system under the bonnet. With the car cold, ensure the fluid level sits between the minimum and maximum marks; do not open a hot pressurised cap.

3. 3

   Inspect high-voltage battery cooling vents

   Many hybrids have a cooling air intake vent on the side of the rear passenger seat or under the rear bench. Ensure jackets, bags, pet blankets or thick dust are not blocking this grille, as poor airflow causes the main battery to overheat.

4. 4

   Perform a clean shutdown cycle

   Park the car, switch the ignition fully off, step out, lock the doors with the key fob and wait five minutes. This allows the high-voltage contactors to disengage and the onboard computers to reboot cleanly.

Typical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. Prices depend mainly on whether the fault lies in standard 12-volt electronics or requires specialist labour and modules inside the sealed high-voltage battery pack.

When to go to a garage

• The car displays a red warning symbol, sounds a continuous chime or will not show the green READY light.
• You notice a burning odour, steam or hot electrical smells coming from the engine bay or battery compartment.
• The vehicle drops into severe limp mode and limits your road speed on public carriageways.
• The warning returns immediately after turning the vehicle off and back on.

Questions drivers ask

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.