C1789 — Stop Lamp Relay Output Circuit Open

C1789 — Stop Lamp Relay Output Circuit Open. C1789 identifies an open stop lamp relay output circuit that can leave the brake lamps without a valid signal.

For Ford vehicles

stop lamp relay output circuit – open circuit

vehicle

Is it safe to drive?

Stop driving if the brake lamps are inoperative C1789 identifies an open stop lamp relay output circuit that can leave the brake lamps without a valid signal. Do not continue driving if the lamps do not illuminate, particularly at night or in heavy traffic.

Typical symptoms

  • C1789 is stored or pending in the vehicle control modules.
  • The rear brake lamps may fail to illuminate when the brake pedal is pressed.
  • One or more stop lamps can remain dark depending on where the circuit opens.
  • Brake lamp operation may become intermittent when a loose connector or damaged wire is disturbed.
  • A scan tool may show a brake-lamp command without the expected lamp-current response where that data is supported.

Common causes

  • A failed stop lamp relay or damaged relay socket can leave the output circuit unpowered.
  • An open or damaged wire can interrupt power between the stop lamp relay and the brake lamps.
  • Corroded, loose, heat-damaged, or bent terminals can create an open connection in the relay or lamp harness.
  • A poor ground at the brake lamp assembly or chassis can make the monitored output appear open.
  • A blown fuse, fusible link, or other circuit-protection device can interrupt the stop lamp relay output.
  • A bulb or socket with excessive resistance can make the output appear open on designs that monitor lamp current.

How to diagnose with an OBD scanner

  1. Confirm C1789 with a scan tool Connect an OBD2 scanner using an ELM327 adapter or AIScanAuto, read stored and pending codes, and confirm C1789. Record freeze-frame information and related codes before clearing anything.
  2. Inspect circuit protection and the relay Using the vehicle wiring diagram, check applicable fuses and fusible links. Inspect the stop lamp relay and socket for incorrect seating, corrosion, spread terminals, overheating, or damaged pins.
  3. Trace the relay output wiring Follow the wiring from the relay output toward the brake lamps. Look for chafing, broken conductors, disconnected plugs, water intrusion, pin damage, and sections with insufficient continuity.
  4. Measure the relay circuit Determine from the wiring diagram whether the relay output supplies voltage or switches ground. Test relay input and output while the brake lamps are commanded, and check voltage drop or continuity through the suspect wiring.
  5. Check lamps, sockets, and grounds Verify every affected stop lamp bulb, socket, and ground connection. Correct high resistance, poor grounds, or damaged terminals before condemning the relay.
  6. Repair and verify operation After correcting the open circuit, clear codes with the scan tool only when no active fault remains. Cycle the ignition, confirm all brake lamps illuminate, and verify that C1789 does not return.

Approximate repair cost

A rough independent-shop estimate is USD 120-500 for diagnosis and a typical relay, connector, fuse, or wiring repair. This is an estimate, not a verified quote; final cost varies by region, labor rate, and parts availability.

Common questions

What does OBD2 code C1789 specifically mean?

C1789 means the control module detected an open circuit in the stop lamp relay output, which can prevent the vehicle's brake lamps from illuminating normally.

Can I safely drive with C1789 stored?

Do not drive if C1789 has left the brake lamps inoperative, because drivers behind you may not see your vehicle slowing or stopping. Confirm lamp operation before continuing.

Can an ELM327 or AIScanAuto scanner diagnose C1789?

An ELM327 or AIScanAuto scanner can confirm C1789 and display related stored or pending codes, but it cannot by itself identify the broken wire, bad ground, or faulty connection.

What should be checked first when C1789 is present?

Start with the stop lamp relay, its socket, applicable fuses, and the wiring leading to the brake lamps. Use the vehicle wiring diagram to verify each electrical test.

C1789 — C1789 — Manufacturer Controlled DTC: Causes, Symptoms & Fix

When your vehicle's computer detects a problem with a sensor or actuator circuit that doesn't match what it expects to see, it logs fault code C1789. This particular code falls under the "manufacturer controlled" category, meaning the specific component and circuit it monitors varies from one vehicle make and model to the next. Rather than describing a single failure mode, C1789 serves as a general flag that tells you something is wrong with an electrical circuit the Powertrain Control Module (PCM) is watching.

The PCM constantly monitors voltage, resistance, and signal patterns from sensors and actuators throughout the drive cycle. When these readings drift outside the factory-defined acceptable window for longer than programmed thresholds allow, the module stores C1789 and illuminates the check engine light. At that moment, the computer also captures freeze frame data—a snapshot of engine RPM, load percentage, coolant temperature, vehicle speed, and other sensor readings. That snapshot is gold for diagnosis because it shows exactly what conditions triggered the fault.

Common Symptoms of C1789

  • Check Engine Light illuminated
  • The affected system operates intermittently or stops working entirely
  • Vehicle may enter limp-home mode with reduced power output
  • Fuel consumption may increase due to compensation strategies
  • Traction control or stability systems could be disabled in some vehicles
  • In rare cases, additional safety-related systems trigger warnings

What Causes C1789?

  • Damaged, corroded, loose, or broken connector at the sensor or actuator
  • Open or shorted wiring somewhere in the circuit
  • Blown fuse feeding power to the affected circuit
  • Failed control module unable to drive the circuit properly
  • Grounding issues causing erratic signal behavior
  • Failed sensor or actuator producing out-of-range readings
  • Aftermarket accessories interfering with factory circuit operation

How to Diagnose C1789

Start by connecting an OBD2 scanner and confirming C1789 is stored. Save the freeze frame data immediately—this information tells you engine speed, load, temperature, and vehicle speed at the exact moment the fault occurred. Replicate those conditions during your test drive to trigger the problem.

Check for related diagnostic trouble codes. P-codes, B-codes, and other C-codes often point directly to the failing component. Many manufacturers publish technical service bulletins (TSB) that describe known fixes for C0xxx codes, including software updates or revised part numbers. Search the NHTSA database or your manufacturer's website using your VIN.

Perform a thorough visual inspection of the wiring harness and connectors associated with the affected circuit. Look for damaged insulation, broken wires, corrosion buildup, or any signs of moisture intrusion. Poor connections at the sensor or actuator itself are surprisingly common.

Use a multimeter to check reference voltage, ground integrity, and signal continuity at the component connector. Compare your readings against factory specifications. Low voltage or poor ground will cause the PCM to set C1789 even when the sensor appears functional.

Finally, perform a wiggle test on the harness while monitoring live data. Any dropouts or signal interruptions point to broken wires inside the insulation that only fail under movement. Clear the code, complete a full drive cycle under the original freeze frame conditions, and verify C1789 does not return.

Repair Options and Costs

If the wiring harness or connector is damaged, repair typically runs $80–$250 for parts and labor at an independent shop. Soldering a broken wire or replacing a corroded connector is usually straightforward.

When the sensor or actuator itself has failed, replacement costs between $150–$500 depending on the part and labor time required. OEM sensors tend to cost more than aftermarket alternatives but often last longer.

In cases where the control module cannot properly drive the circuit, repair or replacement ranges from $300–$1,000. Some modules can be sent out for repair at the lower end of that range; others require full replacement and programming.

Overall, C1789 repairs typically cost $80–$1,000 depending on the root cause. The most expensive scenario—control module replacement—should be your last resort after ruling out simpler issues like wiring damage or a bad sensor.

Can You Drive with C1789?

In most cases, the vehicle remains driveable with C1789 stored, though you may experience reduced performance or lose functionality in auxiliary systems. Treat any stalling, failure to start, or warnings related to brakes and steering as urgent and stop driving immediately.

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