U0183 is a generic OBD-II network communication code indicating that other vehicle control modules have lost expected communication with the Rear Lighting Control Module (LCM-R). The fault may involve CAN-bus wiring, module power or ground, connector problems, or an internal module failure.
1. Quick Diagnostic Answer
Before replacing the rear lighting module, determine whether the module is:
- Completely offline
- Intermittently communicating
- Powered but unable to communicate
- Communicating normally but storing U0183 as a history code
Start with a full vehicle scan, battery and charging-system check, module power and ground testing, and CAN-bus testing.
2. System / Module Function
The Rear Lighting Control Module manages or coordinates rear lighting functions depending on the vehicle design.
- Tail lamps
- Brake lamps
- Turn signals
- Reverse lamps
- Rear fog lamps
- License-plate lighting
- Lighting diagnostics
- Communication with the BCM and other modules
The exact module name, location, number of connectors and controlled functions vary by manufacturer.
3. Symptoms
- Rear lights inoperative or partially inoperative
- Turn-signal or brake-light problems
- Lighting warning message
- Multiple U-codes
- Body-control communication faults
- Scan tool unable to communicate with the rear lighting module
- Intermittent lighting operation
- U0183 returning after clearing
If required rear lighting is not operating correctly, avoid normal road use until the system is repaired.
4. Common Causes
- Open CAN-H or CAN-L circuit
- Short between CAN circuits
- CAN circuit shorted to power or ground
- Loss of module battery supply
- Poor module ground
- Blown fuse
- Corroded connector
- Loose or backed-out terminal
- Water intrusion
- Damaged wiring near the rear body harness
- Poor connector terminal tension
- Low system voltage
- Excessive charging-system voltage or electrical noise
- Faulty Rear Lighting Control Module
A faulty module should normally be considered after power, ground, wiring and network communication have been verified.
5. Severity
Severity: High when rear safety lighting is affected.
The communication code itself does not prove that every rear-light function has failed. Severity depends on the functions controlled by the module and whether the vehicle still has operational brake, turn, tail and other required lighting.
6. Repair Cost
Actual cost depends heavily on vehicle design.
- Diagnostic testing: $50–$150
- Connector/wiring repair: $80–$300
- Fuse or power/ground repair: $20–$150
- Lighting module replacement: $200–$800+
- Programming/configuration: $50–$250+
These are general estimates, not manufacturer-specific prices.
7. Diagnostic Procedure
Step 1 – Perform a Full Module Scan
Scan all available control modules, not only the BCM.
Record:
- U0183 status
- Other U-codes
- Battery or voltage-related codes
- CAN/network codes
- Communication status of the Rear Lighting Control Module
If several unrelated modules simultaneously report communication failures, investigate the network or power supply before focusing exclusively on the rear lighting module.
Step 2 – Check Battery and Charging Voltage
A low or unstable electrical supply can create communication faults.
With the engine off, a healthy fully charged 12-V battery will commonly be around 12.4–12.7 V.
With the engine running, many conventional systems operate around 13.5–14.8 V, but the exact charging strategy is vehicle dependent.
Do not condemn the alternator solely because voltage is outside these approximate values; smart-charging systems can intentionally vary voltage.
8. Multimeter & Electrical Testing
Use the OEM wiring diagram to identify the exact connector pins.
Do not assume a universal pin number or wire color.
Module Power
Set the multimeter to DC volts.
Black probe → known-good ground
Red probe → module B+ supply
You should normally find voltage close to battery/system voltage on a powered B+ circuit.
Module Ground
Red probe → battery positive
Black probe → module ground circuit
A reading close to battery voltage indicates that the ground circuit can carry voltage under the test condition.
For a more accurate ground diagnosis, perform a voltage-drop test under load rather than relying only on continuity.
CAN Network
Identify CAN-H and CAN-L from the vehicle wiring diagram.
Typical high-speed CAN systems use approximately:
- CAN-H ≈ 2.5 V at recessive state, rising toward approximately 3.5 V during dominant bits
- CAN-L ≈ 2.5 V at recessive state, falling toward approximately 1.5 V during dominant bits
These are general reference values, not universal OEM specifications.
CAN Resistance
With the vehicle powered down and the network placed in the correct condition for the test, measure between CAN-H and CAN-L.
A common high-speed CAN network will measure approximately:
60 Ω
This normally represents two 120-Ω termination resistors in parallel.
However, the exact procedure and expected resistance depend on the vehicle network architecture.
9. Live Data Analysis
If the scan tool can communicate with the Rear Lighting Control Module, enter the module's data stream rather than relying only on generic engine PIDs.
Depending on the vehicle, useful parameters may include:
- Module communication status
- Network status
- Supply voltage
- Rear lamp command/status
- Brake-light command
- Turn-signal command
- Tail-lamp status
- Diagnostic status
- Module internal fault status
What to Look For
A healthy system should show logical changes when the corresponding lighting control is operated.
Brake pedal released → Brake command OFF
Brake pedal pressed → Brake command ON
If the module reports correct commands but the physical lamp does not respond, investigate the output circuit and lighting assembly.
If the module disappears from the scan tool entirely, return to:
Power → Ground → CAN → Connector → Module
10. Oscilloscope / Signal Analysis
An oscilloscope is useful when the network fault is intermittent or when a multimeter shows apparently normal voltage.
Probe CAN-H and CAN-L according to the vehicle wiring diagram.
A healthy CAN signal should show complementary activity:
CAN-H: approximately 2.5 V → higher during dominant bits
CAN-L: approximately 2.5 V → lower during dominant bits
Possible Failure Patterns
- Missing communication
- Distorted CAN-H waveform
- Distorted CAN-L waveform
- Shorted CAN-H/CAN-L levels
- Excessive electrical noise
- Intermittent dropouts
- Reflections caused by termination or wiring problems
If the waveform changes when the rear harness or connector is gently moved, investigate that section for an intermittent open, poor terminal connection or damaged wiring.
11. Common Diagnostic Mistakes & After-Repair Verification
Common Diagnostic Mistakes
Replacing the module first:
U0183 does not automatically mean the module has failed.
Ignoring power and ground:
A module without stable power cannot communicate regardless of its internal condition.
Assuming wire colors:
Wire colors vary significantly between manufacturers and models.
Using universal pin numbers:
Connector pin locations must come from the vehicle-specific wiring diagram.
Ignoring other U-codes:
Multiple communication codes can indicate a larger network or voltage problem.
Clearing the code without testing:
The code may return immediately if the communication fault remains.
After Repair
- Clear all related DTCs.
- Cycle the ignition.
- Perform another full-module scan.
- Confirm communication with the Rear Lighting Control Module.
- Operate the rear lighting functions.
- Check live data/status where available.
- Perform a road test if appropriate.
- Rescan for returning U-codes.
If the replacement module requires programming, configuration or calibration, complete the manufacturer's procedure before final verification.
12. FAQ + Related Codes + OEM Note
Can a weak battery cause U0183?
Yes. Low system voltage can cause control modules to reset or temporarily lose communication. Check battery and charging voltage before condemning the CAN network.
Can an alternator problem cause U0183?
Yes, potentially. Excessive charging voltage, unstable system voltage or abnormal electrical noise can interfere with module operation and communication. It is a possible contributor, not an automatic diagnosis.
Can a bad CAN wire cause U0183?
Yes. An open, shorted, corroded or intermittently connected CAN circuit can prevent the Rear Lighting Control Module from communicating.
Can a multimeter diagnose U0183?
Yes. A multimeter is useful for checking module power, grounds, voltage drop and CAN resistance/voltage. An oscilloscope is preferable for capturing intermittent CAN waveform problems.
Does U0183 require module replacement?
No. Wiring, power, ground, connectors and CAN communication should be verified first.
OEM Note
Connector pin numbers, wire colors, module location, CAN architecture, voltage specifications and diagnostic procedures vary by manufacturer. Always use the vehicle-specific wiring diagram and OEM service information before testing individual pins.
Related U-Codes
- U0181 – Lost Communication With Dynamic Headlight Leveling Control Module
- U0182 – Lost Communication With Lighting Control Module – Front
- U0183 – Lost Communication With Lighting Control Module – Rear
- U0184 – Lost Communication With Radio
- U0185 – Lost Communication With Climate Control Module
- U0186 – Lost Communication With Audio Amplifier
Final Diagnostic Principle
For U0183, do not start by replacing the lighting module. Establish whether the failure is caused by power, ground, CAN communication, connector/wiring, or the module itself. This approach helps prevent unnecessary module replacement and provides a reproducible diagnostic path.