Comprehensive diagnostic guide for OBD-II code C1116

Disclaimer on data

• C1116 is a chassis (C) category code in OBD-II. In the limited NHTSA real-world data provided here, at least one complaint specifically linked C1116 to a stop lamp (brake light) circuit problem. This guides the leading suspect category for C1116, but other chassis/Brake/ABS-related issues are possible (as with many OBD-II C codes).
• For general context, OBD-II diagnostic trouble codes (DTCs) are standardized, with C codes representing chassis problems. This framework is described in general OBD-II literature and is referenced in the technical summaries .

What This Code Means

• C1116: In the observed real-world complaint, the fault was described as a stop lamp (brake light) circuit problem. The data suggests the C1116 code can be associated with issues in the brake light circuit or stop lamp switch/inputs to the vehicle's body control/ABS controller. Treat C1116 as a chassis-related fault that commonly involves the brake-light circuit or its sensing input.
• If the code is accompanied by ABS illumination or brake warning lights, consider both brake-light circuits and ABS/chassis control inputs to the ABS module as possible sources, and verify wiring/contacts to the stop lamp switch and tail-lamp assemblies.

Symptoms

• ABS warning light on, possibly along with the brake warning light.
• Brake/stop lamps not functioning properly (dim, intermittent, or inoperative).
• Customer complaint may mention brake pedal feel or misreading of brake lamp behavior as a brake issue.
• Vehicle may fail a basic safety inspection due to faulty brake lights or ABS warning.

Diagnostic Approach

1) Gather data and confirm the code

• Confirm the exact DTC and note any freeze-frame data.
• Look for other codes (P, B, U, or additional C codes) that could point to related modules (BCM, ABS, or PCM).

2) Visual and functional inspection

• Inspect all brake/tail lamp assemblies for corrosion, moisture, damaged bulbs, blown fuses, and missing or loose connectors.
• Check the stop lamp switch/ brake pedal switch mounting and condition. Make sure the switch is actuating properly when the pedal is depressed.
• Check wiring harnesses and connectors between the tail lamps, stop lamp switch, and the ABS/BCM control modules for damage, pin corrosion, or mis-seated connectors.

3) Verify power and grounds

• With the ignition on (engine off, as applicable per vehicle), test for proper voltage at the stop lamp switch input and the outputs that feed the tail lamps.
• Check grounds for the brake light circuit and the ABS/BCM common grounding points. A poor ground can cause intermittent or dim brake lamps and may trigger related DTCs.

4) Stop lamp switch circuit testing

• Continuity test: Check the brake pedal switch for proper resistance when the pedal is pressed/unpressed.
• Activation test: Verify the switch is providing a signal to the BCM/ABS controller when the pedal is depressed.
• If the stop lamp switch is adjustable, verify proper positioning per service data.

5) Wiring and connector checks

• Inspect for insulation wear, abrasion, or chafing along the brake-light circuit harness; check for shorts to power, shorts to ground, and broken conductors.
• Verify connector integrity on all joints involved in the stop lamp circuit and tail lamp connections.

6) ABS/BCM/related module interactions

• If accessible, check for fault codes in the ABS/BCM that reference brake-light input circuits or stop lamp switch signals.
• Some vehicles use the stop lamp switch input to limbs of the ABS/DSC system; a fault in that pathway can set C1116.

7) Component replacement considerations

• Stop lamp switch: Most often a primary suspect when C1116 is tied to the brake-light circuit.
• Tail lamp assemblies: Replace or repair any assemblies with persistent corrosion or water intrusion if they fail electrical tests.
• Wiring repairs: Replace or repair damaged harness sections, fix corroded pins, and re-seat connectors as needed.
• BCM/ABS module: Only after system-level tests indicate module fault (rarely the first fix, but possible on some makes/models).

8) Verification and validation

• Clear the DTCs after repairs and perform a road test.
• Confirm brake lights function correctly with pedal depression and with the parking brake engaged (as appropriate for the vehicle).
• Re-scan for new or remaining DTCs, especially any C codes, to ensure the system is stable and no other faults exist.

Tools and data you'll typically use

• OBD-II scan tool to read/clear codes and view freeze-frame data.
• Digital multimeter (DMM) to verify voltages, continuity, and resistance on stop lamp switch circuit and related wiring.
• A wiring diagram or service manual for the vehicle to identify stops for the stop lamp switch, tail lamps, fuses, and BCM/ABS input/output pins.
• Light tester or a test lamp to verify tail lamp output during pedal actuation.
• Basic hand tools (screwdrivers, prying tools) for accessing tail lamps, switch, and connectors.

Key safety notes

• When working near the vehicle's electrical system, disconnect the battery if you'll be unplugging/handling airbag-related components or if procedures require it per the service data.
• Brake lights are a critical safety feature; ensure the vehicle is safely immobilized during testing. Use proper PPE to guard against sparks or shorts during wiring checks.
• Verify that the vehicle is in a safe test environment; never rely on brake lights during test drives without a trained observer.

Probable Causes

• Leading suspect: Stop lamp switch circuit failure or brake-light wiring/connectors (supports the real-world complaint associating C1116 with stop lamp issues). 60-70% probability in the absence of other evidence.
• Wiring/ground faults in the stop lamp circuit (insulation damage, corroded grounds, loose connectors): 15-25%.
• Faulty tail lamp assemblies or internal wiring in the lamp housings: 5-15%.
• BCM/ABS input/output issue or a module fault affecting the stop lamp input path: 5-10%.

Documentation

• Exact fault code(s) and any freeze-frame data.
• Symptoms observed (ABS warning light status, brake light operation, any blinking lights, etc.).
• Visual findings (bulbs, fuses, switch mounting, wiring condition).
• Measurements (voltage at switch input/output, continuity across switch, resistance to ground for relevant wires).
• Repairs performed (switch replacement, wiring repair, lamp replacement, module re-flash if performed).
• Verification results (clear codes, road test results, lamp operation confirmation).

Related notes and sources

• NHTSA real-world complaint example: User reports ABS light on; diagnosis linked C1116 to a stop lamp circuit problem. This anchors C1116 as a chassis-related fault tied to brake-light circuitry in at least one case.
• Wikipedia - OBD-II and Diagnostic Trouble Codes: Provides general context on how DTCs are structured (P, B, C, U) and explains that OBD-II uses standardized codes to identify failed or abnormal conditions in automotive systems, with C codes generally associated with chassis-related concerns. This supports the classification and cross-system thinking when diagnosing C1116.

This diagnostic guide was generated using verified reference data:

• NHTSA Consumer Complaints: 1 real-world reports analyzed
• Wikipedia Technical Articles: OBD-II

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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