These instructions apply to the Lexus RX Mk2 (XU30) 2003-2008.
For other models, please choose your vehicle here.
Disconnecting the 12 V battery on your Lexus RX (2003–2008) — the XU30 — for a new battery, winter storage, or work near the airbags catches owners out on the hybrid, because on the RX 400h the 12 V is in the boot, not under the bonnet. Done in the right order, it is a job most owners can manage with a single spanner, and the car relearns the handful of settings it loses within a short drive.
You typically pull the 12 V battery for one of four reasons: the battery itself is tired and won’t hold a charge, you’re storing the RX over winter and want to stop a slow drain flattening it, you’re doing bodywork or electrical work and need the system dead, or you’re recovering after a jump-start and want a clean power-down. In every case the sequence is the same, and the important thing on this generation is knowing which battery you’re dealing with.
Find the 12 V Battery First
The XU30 was sold as a petrol RX 300/350 and as the RX 400h petrol-electric hybrid, and the two put the 12 V starter/accessory battery in different places. Identify your variant before you reach for a spanner — opening the bonnet on a 400h expecting to find the battery there is the classic false start.
- RX 400h (hybrid): the 12 V battery is in the luggage compartment (boot), under the floor/trim. The under-bonnet terminals you can see are jump-start posts only. Never touch the orange high-voltage cabling — that is the traction battery’s wiring and is off-limits; you only ever work on the ordinary 12 V battery.
- RX 300 / RX 350 (petrol): the battery sits in the engine bay in the conventional place, usually under a plastic cover you unclip to reach the terminals.
On both, the negative terminal wears a black cover or marking and a − symbol; the positive is red and marked +. Confirm the markings by eye before you loosen anything.
Getting Ready
- Switch the ignition fully off — on the 400h make sure the car is not in READY and the dash is dark — then take the smart key well away from the car.
- Turn off the lights, climate, radio and anything else that draws power.
- Note down your radio station presets and any security-code details in case you need to re-enter them.
- Wait a minute or two after switching off so the modules power down before you disturb the battery.
- Have the correct spanner (usually 10 mm) ready, plus gloves and eye protection.
Disconnecting Safely
- Switch everything off and confirm the car is not in READY.
- Disconnect the negative (−) terminal first, ease the clamp off the post and tuck the cable aside so it can’t spring back.
- Then disconnect the positive (+) terminal.
- Working near the airbags or seat-belt pretensioners? Wait 90 seconds after disconnecting before you start that work, so the airbag system fully discharges.
Reconnecting and Re-initialising
Reconnect in the reverse order: positive (+) first, then negative (−) last, and nip both clamps up firmly — a loose clamp is the commonest cause of an odd fault afterwards. The RX then relearns a few items as you drive:
- Clock and radio presets — reset the time and re-store your stations.
- Steering angle / stability — drive gently straight for a moment, then turn the wheel fully each way, and the VSC system re-learns its centre.
- Power windows — if one-touch/auto is lost, pull the switch up and hold at the fully-closed position for a moment to re-teach the window.
If Something Isn’t Right Afterwards
- Check the clamps are tight before assuming anything is wrong — most post-disconnect gremlins are a loose terminal.
- Warning lights such as VSC usually self-clear once the car sees a short, steady drive.
- Redo the window and steering relearn if auto-up or the stability light hasn’t settled.
- The gear lever won’t leave ‘P’? Use the console shift-lock release — see our RX shift-lock release guide.
- A light that stays on after a drive has stored a code — look it up on autodtcs.com to see what it means before spending money.
Frequently Asked Questions
Where is the 12 V battery on a Lexus RX 400h?
In the boot (luggage compartment), under the floor trim. The posts under the bonnet are only jump-start points. The orange high-voltage cabling is separate and must never be touched — you only work on the 12 V battery.
And on the RX 300 / 350?
In the engine bay in the conventional place, usually under an unclip cover. No hybrid system to worry about on the petrol cars.
Which terminal do I take off first?
Negative (−) off first when disconnecting, and reconnect it last. On the way back, positive (+) goes on first, negative last. This order keeps you from shorting a spanner against the bodywork.
What needs resetting after I reconnect?
The clock and radio presets, one-touch windows, and the steering-angle/stability learn. Most of it settles on a short drive; the windows may need a quick pull-and-hold to restore auto-up.
Does a new 12 V battery need coding on this RX?
No. A like-for-like 12 V battery on the XU30 is a straight swap — there’s no battery registration step. Just fit it, reconnect in the right order and let the car relearn its settings.
Will disconnecting the battery clear my fault codes or service light?
It can drop temporary codes and reset some trip data, but a fault that’s still present will simply log again and relight the warning. Disconnecting is not a proper service-light reset or a fix for a real fault — find the cause instead.
Independent guide — not affiliated with Lexus. Always follow your official service manual and safety precautions. Read our full disclaimer.