C1787 — Stop Lamp Relay Short to Positive Supply

What does C1787 specifically identify?

C1787 identifies a stop lamp relay output circuit that the vehicle has detected as shorted to a positive supply, such as battery voltage or a fused +B feed.

Can I drive with C1787 stored?

If the brake lamps work correctly, drive cautiously to a repair shop. If they remain on or fail to illuminate, avoid continued driving because rear brake lighting is safety-critical.

What should I check first for C1787?

Use an ELM327 scanner or AIScanAuto to confirm C1787, then inspect the stop lamp relay, socket, wiring, and any aftermarket lamp connections for an unintended short to positive supply.

Does C1787 always mean the relay must be replaced?

Not necessarily. Isolate the relay and test the circuit first, because damaged wiring, a bridged socket, contaminated terminals, or an aftermarket harness can create the same C1787 condition.

When your scan tool displays C1787, it means the vehicle's control module has detected a signal or circuit behavior that falls outside its expected range. The code is classified as manufacturer-controlled, meaning the specific failure it represents can vary significantly depending on which system triggered the fault. Unlike generic powertrain codes, C1787 draws its exact definition from the automaker's software logic, so the affected component differs from one vehicle to the next.

The module does not illuminate the check engine light after a single bad reading. It requires the fault condition to occur repeatedly under specific operating parameters—engine load, rpm, coolant temperature, vehicle speed—before logging C1787 and activating the warning indicator. The freeze frame data stored alongside the code captures those exact conditions, giving you a blueprint for recreating the failure during diagnosis.

Common Symptoms

When C1787 is active, you may notice one or more of the following:

• The check engine light or other warning lamp illuminates on the instrument cluster
• The system tied to the fault operates intermittently or stops functioning entirely
• Reduced engine performance or limp-home mode in severe cases
• Noticeably higher fuel consumption compared to normal driving
• Specific auxiliary systems—such as traction control, stability programs, or climate functions—may be disabled or behave erratically

Typical Causes

The root cause of C1787 usually falls into one of these categories, listed from most to least frequent:

• Damaged or corroded wiring and connectors interrupting signal or power delivery
• A poor or loose ground connection starving the circuit of proper reference voltage
• A burned-out fuse protecting the affected circuit
• A failed control module driver unable to properly switch the circuit
• An open or shorted wire within the harness causing unpredictable voltage levels
• A defective sensor or actuator producing incorrect output signals

Diagnostic Procedure

Follow these steps to pinpoint the source of C1787 without wasting time or money on unnecessary parts:

1. Connect a quality OBD2 scanner, record C1787 along with any accompanying codes, and save the freeze frame snapshot—these recorded conditions are the map to your problem.
2. Investigate all stored codes together; related faults often point to the primary failure. Also search manufacturer TSB databases for your make and model—many C0xxx codes have published factory solutions or calibration updates.
3. Perform a thorough visual inspection of the wiring harness and connectors associated with the fault circuit, looking for chafed insulation, broken strands, moisture intrusion, or pins that have backed out of their housings.
4. Use a multimeter to check reference voltage, ground integrity, and signal continuity at the component's connector while the system is powered up.
5. Conduct a wiggle test by gently moving the harness while monitoring live data—any signal dropouts or wild fluctuations expose broken wires hidden inside the insulation.
6. Clear C1787 after repairs, complete a test drive that recreates the freeze frame conditions, and verify the code does not return.

Repair Options and Costs

Address the most accessible and affordable culprits before committing to expensive replacements. Common repair scenarios for C1787 include:

• Wiring harness repair or connector replacement: $80–$250
• Sensor or actuator replacement: $150–$500
• Control module diagnosis and repair: $300–$1,000

These estimates reflect parts and labor at independent shop rates. Total cost to resolve C1787 typically falls between $80 and $1,000, with the exact figure determined by which component fails. Proper diagnosis before purchasing parts prevents the costly mistake of replacing a perfectly good module when the real issue is a corroded ground wire.

Is It Safe to Drive?

Most vehicles remain drivable when C1787 is stored, though you may experience diminished performance or an inoperative auxiliary feature. The risk level depends entirely on which system the code references. If the failure affects critical functions such as braking assistance, power steering, or engine operation to the point of stalling, treat the situation as an emergency and seek repair immediately. Mild symptoms like a disabled climate fan or intermittent gauge behavior allow you to drive carefully to a repair facility, but do not ignore the problem indefinitely.

Related OBD2 Codes

• C1700: Left Rear Sensor Circuit Failure
• C1701: Left Rear Sensor Circuit Fault
• C1702: RightRear Sensor Circuit Short to Vbat
• C1703: Right Rear Sensor Circuit Failure
• C1704: Right Rear Sensor Circuit Fault
• C1705: Left Rear Center Sensor Circuit Short to Vbat