What Does B1978 Mean?
This code indicates that the Body Control Module (BCM) has detected an unexpected voltage on the passenger's front seat down switch circuit, specifically that the circuit is reading battery voltage when it should not be. This suggests the switch circuit is being continuously powered, typically indicating a short to the power supply wire. The BCM monitors seat position switches to control powered seat motors and memory functions.
Common Causes
40%
Damaged or chafed wiring harness under the seat causing short to power wire
30%
Failed seat adjustment switch with internal short to power circuit
20%
Damaged or corroded connector pins at seat switch or BCM causing cross-circuit contact
10%
Failed Body Control Module with internal short circuit on seat switch input
Diagnostic Steps
1
Step 1: Perform visual inspection of passenger seat wiring harness and connectors, checking for chafed insulation, pinched wires, water intrusion, or corroded terminals at seat switch and under-seat connections
2
Step 2: Disconnect the passenger seat adjustment switch connector and measure voltage at the down switch circuit pin (BCM side) using a digital multimeter - should read 0V with ignition on; if battery voltage present, wiring harness has short to power
3
Step 3: If no voltage present at connector, perform resistance test across the down switch terminals with switch disconnected - should show open circuit (infinite resistance) when not pressed; low resistance indicates failed switch
4
Step 4: Check for proper ground circuit continuity at seat switch ground pin to chassis ground - should read less than 1 ohm resistance; high resistance indicates poor ground connection
5
Step 5: If all wiring and switch test normal, check BCM connector terminals for damage, corrosion, or bent pins causing internal short; measure resistance between down switch circuit pin and power supply pins at BCM connector
6
Step 6: After repairs, clear codes, operate seat adjustment through full range of motion, and verify code does not return; confirm proper seat operation in all directions
Estimated Repair Cost
$100 - $600
Parts + labor, varies by vehicle and location
This code indicates that the Body Control Module (BCM) has detected an unexpected voltage on the passenger's front seat down switch circuit, specifically that the circuit is reading battery voltage when it should not be. This suggests the switch circuit is being continuously powered, typically indic...
The most common cause of B1978 (Passenger's Front Seat Down Switch Circuit Short to Battery) is: Damaged or chafed wiring harness under the seat causing short to power wire
Typical repair costs for B1978 range from $100 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This code does not affect vehicle safety or drivability and the vehicle can be operated normally. The passenger seat adjustment function may be disabled or operate incorrectly, but this does not pose a safety risk requiring immediate repair.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1978 to identify the root cause.
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Quick Info
Category
Body
System
Body Control System (Power Seat)
Difficulty
Type
Manufacturer
Recommended Tools
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