Lexus IS: IS 300h 12V Auxiliary Battery Failure

The IS 300h hybrid relies on a dedicated 12V lead-acid auxiliary battery to power control modules, initiate hybrid system communication, and begin the high-voltage startup sequence — the traction battery alone cannot bring the car to the READY state. This auxiliary battery typically fails between four and seven years of age, often without significant prior warning symptoms. When it fails below threshold voltage, the car will not enter the READY state despite pressing the start button, stranding the owner in a scenario that closely resembles a dead high-voltage battery and cannot be resolved by conventional jump-starting techniques.

• Vehicle fails to enter READY state — starter button pressed but no response or error displayed
• Random warning lights, error messages, or system faults appearing intermittently
• Infotainment screen flickering, rebooting, or losing settings unexpectedly
• Clock or personalisation settings resetting after the car has been parked overnight
• Hybrid pre-check sequence taking noticeably longer than normal before READY illuminates

1. 1

   Ask for the age and last replacement date of the 12V auxiliary battery — if it is the original factory battery and the vehicle is over five years old, treat replacement as a near-term cost.

2. 2

   On the test drive, verify the car enters READY state immediately on pressing the start button.

3. 3

   Note any random electrical glitches such as infotainment resets, clock losing time, or intermittent warning lights, which commonly precede auxiliary battery failure.

Parts involved: 12V battery.

$200 to $500 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “When was the 12V auxiliary battery last replaced, and is there a receipt or sticker on the battery?”
• “Has the car ever failed to enter READY state or shown unexpected electrical warning lights?”
• “Is there any service record confirming the auxiliary battery has been load-tested recently?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.