B2172: Inertia Switch Circuit Open
The B2172 code indicates that the powertrain control module (PCM) or security module has detected an open circuit in the inertia switch input. The inertia switch is a safety device that cuts fuel supply in the event of a collision or significant impact. An open circuit means there is no electrical continuity between the switch and the control module, which can prevent the system from correctly recognizing the switch's status. On Ford vehicles, this switch is typically located in the passenger compartment near the center pillar or behind the glove box, and its function is to shut off the fuel pump to prevent post-collision fires.
1. Commonly Affected Makes
2. Most Common Causes
- Inertia switch disconnected or accidentally deactivated
- Damaged or cut wiring between the inertia switch and control module
- Corroded or loose inertia switch connector
- Defective inertia switch or one that has tripped and not been reset
- Faulty control module that monitors the circuit
- Poor ground in the inertia switch circuit
3. Symptoms You Will Notice
- Engine cranks but does not start
- Fuel pump does not run
- Check Engine Light or warning light illuminated
4. Step-by-Step Diagnosis
Check the inertia switch for proper reset (press the reset button if tripped).
Inspect the wiring and connector for damage, corrosion, or looseness.
Test continuity between the inertia switch and the control module.
Verify the ground circuit is intact and clean.
If all else is good, test the inertia switch itself and replace if faulty.
As a last resort, check the control module for faults.
5. Frequently Asked Questions
Is it safe to drive?▼
No, it is not safe to drive because the fuel pump may be disabled, causing the engine to stall or not start, and the safety system may not function properly in a collision.
How much does it cost to fix?▼
Repair costs typically range from $50 to $200, depending on whether it's a simple reset, wiring repair, or switch replacement.