What Does B1578 Mean?
This code indicates the Body Control Module (BCM) has detected a short to ground in the parking lamp input circuit. The BCM monitors voltage on the parking lamp switch input circuit and has detected an abnormally low voltage (near 0 volts), indicating the circuit is shorted to ground somewhere between the switch and the BCM. This prevents proper operation of the parking lamp system and may cause lamps to remain illuminated or fail to function.
Common Causes
45%
Damaged or chafed wiring harness causing short to ground in parking lamp circuit
30%
Faulty parking lamp switch with internal short to ground
15%
Corroded or water-damaged BCM connector pins causing short circuit
10%
Failed Body Control Module with internal circuit damage
Diagnostic Steps
1
Step 1: Perform visual inspection of parking lamp wiring harness from switch to BCM, looking for pinched, chafed, or damaged wires especially near body panels, doors, and routing clips. Check for any obvious signs of wire damage or exposed copper.
2
Step 2: Disconnect the parking lamp switch connector and measure resistance between the switch input circuit wire (BCM side) and ground using a digital multimeter. Reading should be infinite (OL/open circuit). If reading is less than 10 ohms, wiring harness has a short to ground.
3
Step 3: If wiring tests good, test parking lamp switch by measuring resistance across switch terminals with multimeter in all switch positions per manufacturer specifications. Replace switch if out of specification or showing continuity to ground when it should not.
4
Step 4: Inspect BCM connector for corrosion, pushed-out pins, or water intrusion. Disconnect BCM and measure resistance between parking lamp input pin at BCM connector and ground. Should read infinite resistance. If shorted, BCM is likely faulty.
5
Step 5: After repair, clear codes and cycle parking lamps through all positions multiple times. Monitor live data for parking lamp switch status to confirm proper voltage levels (typically battery voltage when off, ground when activated). Verify code does not return.
Estimated Repair Cost
$75 - $600
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a short to ground in the parking lamp input circuit. The BCM monitors voltage on the parking lamp switch input circuit and has detected an abnormally low voltage (near 0 volts), indicating the circuit is shorted to ground somewhere betwe...
The most common cause of B1578 (Lamp Park Input Circuit Short To Ground) is: Damaged or chafed wiring harness causing short to ground in parking lamp circuit
Typical repair costs for B1578 range from $75 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Generally safe to drive during daylight hours, but parking lamps may not function properly reducing visibility to other drivers at night or in low-light conditions. Repair within a few days to maintain safe vehicle lighting operation and avoid potential traffic citations.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1578 to identify the root cause.
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Quick Info
Category
Body
System
Exterior Lighting System
Difficulty
Type
Manufacturer
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