P1787

What it means

On Volkswagen, P1787 means the control unit has detected a short to ground in the reversing light circuit. It sets when the monitored circuit voltage is pulled to ground when the reverse light output or switch signal should be present or high. The exact circuit and which module monitors it depend on the model, gearbox type and year.

Symptoms

• Reversing lights not working or staying off when reverse is selected
• Reversing lights staying on in other gears
• Blown reversing light fuse
• Warning light or fault message on the dashboard
• Reverse-related functions such as parking sensors or reversing camera not activating (model dependent)

Possible causes

• Chafed or damaged wiring to the reversing lights shorting to bodywork or earth, often at the boot hinge or tailgate loomlikely
• Water ingress or corrosion in a rear light cluster, bulb holder or connector causing a short to groundlikely
• Faulty reversing light switch on the gearbox (manual) or faulty selector or range sensor (automatic)possible
• Wrong or faulty bulb, or a bulb base shorting internallypossible
• Damaged connector pins or wiring at the gearbox switch from heat, oil or road debrispossible
• Fault within the control unit that drives or monitors the reversing light circuitrare

How to diagnose

1. Read all stored codes and freeze frame data, note whether the fault is current or intermittent, then check for related codes in other modules
2. Check the reversing light fuse and bulbs, and confirm the correct bulb type is fitted
3. Select reverse with the ignition on and see whether the lights work, stay on or blow the fuse
4. Inspect the rear light clusters, bulb holders and connectors for water, corrosion, melting or burnt contacts
5. Inspect the wiring through the tailgate or boot hinge area and along the route to the gearbox for chafing
6. Check the gearbox reversing switch or selector sensor connector and wiring for damage, oil contamination or corrosion
7. Using the wiring diagram for the specific model, test the circuit for a short to ground with the connectors disconnected and the circuit isolated
8. Check live data for the reverse gear signal to see whether it matches the selected gear

How to fix

• Repair or replace the damaged wiring section and secure it away from sharp edges
• Clean, dry or replace corroded connectors and bulb holders, and cure the water leak at its source
• Replace the faulty reversing light switch or selector sensor, following the model-specific procedure
• Replace faulty or incorrect bulbs and any blown fuse once the short is cleared
• Repair or replace the control unit only after wiring, switches and loads are confirmed good

Can I keep driving?

This code for other makes

• P1787Ford2-1 Downshift Error
• P1787Lincoln2-1 Downshift Error
• P1787Mercury2-1 Downshift Error
• P1787Other2-1 Downshift Error

Nearby codes

• P1786VolkswagenReversing Light Circuit Open
• P1788VolkswagenReversing Light Circuit Short To B+