P1815

What it means

P1815 on a Mercury means the transmission control module has detected a short to ground in the 4-wheel drive mode select circuit. The module monitors the signal from the 4WD mode selector (switch or transfer case selector input) and has seen a voltage pulled to ground when it should not be. Exact circuit layout and the type of selector vary by model, so check the wiring information for the specific vehicle.

Symptoms

• Transmission or 4WD warning light on the dashboard
• Malfunction indicator lamp illuminated
• 4WD mode selector not responding or not engaging the selected mode
• Transmission stuck in a default or limp-type shift strategy on some models
• Harsh or unusual shifts
• 4WD indicator lamps flashing or showing the wrong mode

Possible causes

• Damaged, chafed or pinched wiring in the 4WD mode select circuit shorting to groundlikely
• Faulty 4WD mode selector switch or control with an internal shortlikely
• Corroded, wet or damaged connector at the selector, transfer case or transmission harnesspossible
• Faulty transfer case shift motor or position sensor circuit pulling the signal to ground (model dependent)possible
• Aftermarket accessory wiring or previous repair damaging the harnesspossible
• Faulty transmission control module or related control unitrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, then check for other 4WD or transfer case codes
2. Check for relevant technical service bulletins for the specific model and engine
3. Visually inspect the 4WD selector wiring and connectors for chafing, water ingress, corrosion and previous repairs
4. Operate the selector through every position and watch the live data for the mode select input to see if it changes or stays low
5. Unplug the selector and check whether the signal state changes; if the short remains, the fault is in the wiring, if it clears, suspect the selector
6. With the battery disconnected and the connectors apart, test the signal wire for continuity to ground using the wiring diagram
7. Check the transfer case shift motor and position sensor circuits for faults, following the model-specific procedure
8. Only after wiring and components pass, consider testing the control module's input and power and ground supplies

How to fix

• Repair or replace the shorted or chafed wiring and secure it clear of sharp edges and heat
• Replace the faulty 4WD mode selector switch
• Clean, repair or replace damaged or corroded connectors and terminals
• Replace a faulty transfer case shift motor or sensor if it is found to be pulling the circuit to ground
• Correct any faulty aftermarket or previous repair wiring, then clear the code and road test to confirm it does not return

Can I keep driving?

This code for other makes

• P1815BuickTransmission Range Switch Start In Wrong Range
• P1815CadillacTransmission Range Switch Start In Wrong Range
• P1815ChevroletTransmission Range Switch Start In Wrong Range
• P1815FordTransmission 4-Wheel Drive Mode Select Short Circuit To Ground
• P1815GMTransmission Range Switch Start In Wrong Range
• P1815GMCTransmission Range Switch Start In Wrong Range
• P1815LincolnTransmission 4-Wheel Drive Mode Select Short Circuit To Ground
• P1815OldsmobileTransmission Range Switch Start In Wrong Range
• P1815OtherTransmission 4-Wheel Drive Mode Select Short Circuit To Ground
• P1815SaturnTransmission Range Switch Start In Wrong Range
• P1815VolkswagenPressure Control Solenoid 1 Short To B+

Nearby codes

• P1812MercuryTransmission 4-Wheel Drive Mode Select Circuit Failure
• P1819MercuryTransmission Neutral Safety Switch Short Circuit To Ground