What Is the Inertia Fuel Cutoff Switch?
An inertia switch (also called a fuel cutoff switch or crash cutoff switch) is a safety device fitted to many vehicles — particularly Ford and some other makes — that cuts power to the fuel pump when it detects a sudden deceleration event, such as a collision. The logic is simple: if the car has crashed, cutting the fuel pump reduces the risk of fire from a ruptured fuel line.
Once triggered, the switch stays in its cut-off state until manually reset. This means the car can be otherwise undamaged but simply won't start after even a minor impact, fender bender, or hitting a kerb hard.
Vehicles Most Likely to Have This Switch
Ford vehicles (Falcon, Ranger, Territory, Focus, Mondeo) are the most common. Some Mazda, Land Rover, and older American vehicles also have them. Japanese imports generally do not, using ECU-based immobiliser functions instead.
How to Tell If the Inertia Switch Has Tripped
- The car was involved in any impact, even a minor one
- The engine cranks normally but won't start, and there are no fault codes
- The fuel pump can't be heard priming when the ignition is turned to "on"
- Everything else on the car appears to be functioning normally
How to Reset the Inertia Switch
The switch is typically located:
- In the boot (behind the carpet, on the side wall) — most common on Ford models
- Under the dashboard on the driver's or passenger's side
- In the engine bay on some models
Consult your owner's manual — Ford manuals include the exact location and reset procedure.
To reset: Press the button on top of the switch firmly until you feel/hear it click back into position. Turn the ignition on and listen for the fuel pump priming. Try to start the car.
Important Warning Before Resetting
Do not reset the switch if:
- You can smell petrol strongly (possible fuel line rupture)
- You can see fuel leaking from anywhere under the car
- The accident was significant — have the car inspected before restarting
The inertia switch exists for safety. If you're in any doubt, have the vehicle towed to a workshop for inspection before attempting a restart.
When the Switch Isn't the Problem
If the switch resets correctly but the car still won't start after an accident, the collision may have caused other damage: a dislodged sensor connector, damaged wiring, a fuel line impact, or structural damage affecting other systems. A workshop inspection is warranted.
Cost
Resetting the switch yourself: free. If a mechanic does it during a post-accident inspection: typically included in inspection costs or billed as minimal labour. Replacing a faulty inertia switch: $60–$150 NZD.