P1900 Coolant Level Lamp Circuit Short to Ground
The P1900 code indicates a short-to-ground fault in the coolant level indicator lamp circuit. This circuit monitors the engine coolant level and triggers a warning on the instrument panel when the level is low. A short to ground means the wiring has lost insulation and is contacting the chassis or ground, or there is an internal fault in the control module that keeps the signal permanently grounded. According to SAE J2012, codes P1900-P1999 are manufacturer-specific, and this code was originally documented by Ford Motor Company for vehicles with a coolant level monitoring system.
01. Most Common Causes
- Coolant level lamp circuit wiring shorted to chassis or ground
- Damaged indicator lamp with internal short to ground
- Coolant level sensor with internal short to ground
- Corroded or moisture-contaminated sensor or lamp connector causing a short
- Instrument cluster control module (ICM) with internal driver circuit failure
- Associated fuse or relay with a ground leak in its control circuit
02. Symptoms You Will Notice
- Coolant level warning light stays on continuously
- Coolant level warning light does not illuminate when coolant is low
- Intermittent or false coolant level warnings
03. Step-by-Step Diagnosis
Verify the coolant level in the reservoir and top off if necessary.
Inspect the wiring and connectors for the coolant level sensor and indicator lamp for damage, corrosion, or moisture.
Check for continuity to ground on the signal wire; if present, isolate the short by disconnecting components.
Test the coolant level sensor and indicator lamp individually for internal shorts.
If all else is normal, suspect a fault in the instrument cluster control module (ICM).
1. Frequently Asked Questions
Is it safe to drive?▼
Yes, but you may lose the low coolant warning. Monitor coolant level manually to avoid overheating.
How much does it cost to fix?▼
Typically $100 to $300, depending on whether it's a wiring repair, sensor replacement, or module replacement.