Severity Medium
B1578

Parking Lamp Circuit Low Voltage

📅 Last reviewed: July 2026
What does the B1578 code mean?

The body control module (BCM, FEM, or SJB depending on the platform) has detected an abnormally low voltage on the parking lamp input circuit, consistent with a short to ground. This circuit is monitored to verify the status of the parking lamps; when the signal remains near 0 volts longer than calibrated, the DTC is set. The condition prevents proper feedback of the parking light circuit status to the control module.

Medium
Severity
Variable
Typical Cost
Medium
Frequency

1. Commonly Affected Makes

FordLincolnMercury

2. Most Common Causes

3. Symptoms You Will Notice

4. Step-by-Step Diagnosis

1

Inspect parking lamp bulbs and sockets for damage or corrosion.

2

Check wiring and connectors for shorts to ground using a multimeter.

3

Test the multi-function switch for internal shorts.

4

Verify BCM operation and voltage reference.

5. Frequently Asked Questions

Is it safe to drive?

Yes, but parking lamps may not function properly, reducing visibility at night. Repair is recommended soon.

How much does it cost to fix?

Typically $100 to $300 depending on the cause and labor rates.

6. Related Codes

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