P1820: 4WD Mode Selector Circuit Short to Ground
The P1820 code indicates a short to ground in the 4WD mode selector circuit. This is triggered when the transmission control module (TCM) or powertrain control module (PCM) detects an abnormally low voltage signal from the 4WD mode switch or sensor, below the expected threshold for normal operation. The system continuously monitors the circuit's resistance and voltage to verify the integrity of the selector, which allows the driver to choose between 2WD, 4WD high, or 4WD low modes.
01. Most Common Causes
- Internal short to ground in the 4WD mode selector switch
- Wiring harness rubbing against the chassis or damaged insulation
- Moisture, corrosion, or bent pins in the 4WD switch connector
- Short circuit in the harness between the selector and control module
- Internal failure of the transfer case control module or 4WD module
- Excessive resistance in the circuit ground path
02. Symptoms You Will Notice
- Check Engine Light or 4WD warning light illuminated
- Inability to shift between 4WD modes or stuck in one mode
03. Step-by-Step Diagnosis
Scan for codes and verify P1820 is present; check for related codes.
Visually inspect the 4WD selector switch and wiring for damage, corrosion, or chafing.
Test the switch circuit for continuity and resistance; compare to manufacturer specs.
Check for shorts to ground using a multimeter at the switch and module connectors.
If wiring and switch are good, suspect a faulty control module.
1. Frequently Asked Questions
Is it safe to drive?▼
You can drive short distances, but avoid off-road or demanding conditions. The vehicle may be stuck in one mode, which could affect handling or drivetrain components.
How much does it cost to fix?▼
Repair costs range from $100 to $400, depending on whether it's a simple wiring repair or a switch/module replacement.