Hybrid won't go to READY? Test the 12-volt battery first

What it means

The 12-volt battery is an ordinary small battery that wakes the car up and powers its control units. If it is flat or worn, the car may refuse to go to READY at all, even with a healthy traction battery.

How a hybrid keeps the 12-volt battery charged

A hybrid usually has no conventional alternator. A DC/DC converter keeps the 12-volt battery charged from the high-voltage side.

A failing traction battery shows up differently, with reduced electric assist, warning lights and reduced performance.

Common causes

• A flat or worn 12-volt battery
• A worn 12-volt battery that cold weather has finally exposed
• A dim dashboard on a car that will not wake up, which points at the 12-volt battery
• The traction battery, although when it fails it shows as reduced electric assist, warning lights and lower performance

What to do now

1. Look at the dashboard when you press the start button. A dim dashboard on a dead car points at the 12-volt battery.
2. Have the 12-volt battery tested first.
3. Find where it sits in the manual: under the hood, under a trunk floor panel or beneath a seat, depending on the model.
4. Jump-start only the 12-volt system, and only at the connection points the manual gives. On many hybrids those are not the battery posts.
5. Test or replace the battery following the manual's procedure, which on some hybrids differs from a conventional car.
6. If reaching the battery means removing a panel that exposes orange cabling, stop there.

12-volt battery or traction battery?

• The car will not go to READY and the dashboard is dim or dark: the 12-volt battery, so have it tested first.
• The car goes to READY but runs less on electricity: traction battery capacity or charging.
• Reduced electric assist, warning lights and lower performance: the traction battery or hybrid system, so have it diagnosed.

What people often get wrong

That a hybrid which will not start needs a new hybrid battery. Test the 12-volt battery before anyone condemns the traction battery.

When the car must go to a workshop

If the car still will not go to READY with a healthy 12-volt battery, or the hybrid system light is on, it needs a hybrid-qualified technician. The traction battery, the power electronics and the orange cabling are never an owner job.

Read next

• The EV will not wake up: usually the small battery, not the big one
• Car won't start: find the fault by the sound
• Hybrid battery failure signs: what to note, and what it is not

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.