What Does B1878 Mean?
This code indicates the restraint control module (RCM/ACM) has detected a short to battery voltage in the driver's seatbelt pretensioner circuit. The pretensioner is a pyrotechnic device that retracts the seatbelt during a collision to reduce occupant movement. The module detected higher than expected voltage on the circuit, indicating the pretensioner firing circuit is shorted to positive battery voltage rather than remaining at the normal standby voltage level.
Safety Warning
This is a critical safety system fault that disables the driver seatbelt pretensioner and may disable other airbag system components. The vehicle should not be driven except to a repair facility as occupant protection is severely compromised in a collision. Immediate repair is required.
Common Causes
45%
Damaged wiring harness or chafed insulation causing short to power in pretensioner circuit under seat or B-pillar
25%
Water intrusion or corrosion in pretensioner connector causing cross-circuit short
20%
Failed seatbelt pretensioner assembly with internal short circuit
10%
Faulty restraint control module with internal driver circuit failure
Diagnostic Steps
1
Step 1: Perform visual inspection of driver seatbelt pretensioner wiring from B-pillar to under-seat connector, checking for pinched wires, chafing against seat tracks, or damaged insulation that could contact power sources
2
Step 2: Disconnect battery negative terminal and wait 10 minutes for system capacitor discharge, then disconnect driver pretensioner connector and inspect for corrosion, water intrusion, bent pins, or melted terminals
3
Step 3: Using a digital multimeter, measure resistance between pretensioner harness connector pins (vehicle side) and battery positive terminal - reading should be infinite (open circuit); any continuity indicates short to power in harness
4
Step 4: Check resistance across pretensioner device terminals (component side) - should read 2-5 ohms typically per manufacturer specs; verify no continuity between either terminal and ground or case
5
Step 5: If harness and pretensioner test normal, suspect restraint control module internal fault - verify all other pretensioner circuits test normal before condemning module
6
Step 6: After repairs, reconnect all components, clear codes, and perform restraint system self-test using scan tool active tests to verify proper circuit operation and confirm airbag light extinguishes
Estimated Repair Cost
$200 - $1,200
Parts + labor, varies by vehicle and location
This code indicates the restraint control module (RCM/ACM) has detected a short to battery voltage in the driver's seatbelt pretensioner circuit. The pretensioner is a pyrotechnic device that retracts the seatbelt during a collision to reduce occupant movement. The module detected higher than expect...
The most common cause of B1878 (Seatbelt Driver Pretensioner Circuit Short to Battery) is: Damaged wiring harness or chafed insulation causing short to power in pretensioner circuit under seat or B-pillar
Typical repair costs for B1878 range from $200 to $1,200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This is a critical safety system fault that disables the driver seatbelt pretensioner and may disable other airbag system components. The vehicle should not be driven except to a repair facility as occupant protection is severely compromised in a collision. Immediate repair is required.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1878 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Body
System
Supplemental Restraint System (SRS/Airbag)
Difficulty
Type
Manufacturer
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
