U013A – Lost Communication With Trailer Lighting Module

System: Network | Standard: ISO/SAE Controlled | Fault type: General

Definition source: SAE J2012/J2012DA (industry standard)

U013A indicates that one or more control modules on the vehicle network are no longer receiving expected messages from the Trailer Lighting Module. In practical terms, the network sees the trailer lighting controller as “not talking” when it should be. This is a communication DTC, not a direct statement that a trailer lamp, fuse, or wiring at the trailer connector is failed. The exact setting criteria, message timeouts, and which modules report U013A can vary by vehicle, so confirm network topology, connector locations, and pinpoint tests using the correct service information before replacing parts.

What Does U013A Mean?

U013A – Lost Communication With Trailer Lighting Module means the vehicle’s network monitoring detected a loss of communication with the Trailer Lighting Module. SAE J2012 defines the standardized structure for DTCs, and in this case the code is describing a network-level issue: required data messages from the Trailer Lighting Module are missing, invalid, or not acknowledged within the monitor’s expected timeframe. The code does not, by itself, confirm that the module is defective; it only confirms that the communication path (module power/ground, network wiring, connectors, or the module itself) is not functioning as expected at the time the fault was detected.

Quick Reference

  • Subsystem: Vehicle network communication between the Trailer Lighting Module and other control modules.
  • Common triggers: Module offline due to power/ground loss, disconnected connector, network open/short, or repeated message timeouts.
  • Likely root-cause buckets: Wiring/connector faults, power/ground supply issues, network bus integrity problems, Trailer Lighting Module fault, or configuration/software issues (varies by vehicle).
  • Severity: Typically affects trailer lighting control/monitoring; may create safety concerns when towing if trailer lamps are not controlled or not monitored correctly.
  • First checks: Verify battery voltage stability, check fuses/feeds and grounds for the Trailer Lighting Module, inspect connectors for damage/corrosion/loose pins, and scan for additional network DTCs.
  • Common mistakes: Replacing trailer bulbs/connector parts first, condemning the module without verifying power/ground and network wiring, or ignoring related communication codes that point to a broader network issue.

Theory of Operation

The Trailer Lighting Module (where equipped) communicates with other vehicle modules over the vehicle network. It typically reports status information (such as output state, fault detection, or trailer presence logic) and receives commands (such as turn, stop, tail, or hazard requests) as network messages. Other modules expect these messages at regular intervals and may also perform basic “presence” checks to confirm the module is online.

U013A sets when the receiving module(s) determine that expected messages from the Trailer Lighting Module are missing or not plausible for long enough to exceed a communication timeout strategy. Causes are commonly loss of module power/ground, an unplugged or poor connection at the module, network wiring faults (open, short, high resistance), or a module that is not operating correctly. Monitoring strategy and exact message handling vary by vehicle.

Symptoms

  • Trailer lights inoperative when commanded by vehicle lighting functions.
  • Trailer light warnings or lamp-out messages related to towing/trailer lighting (if the vehicle supports monitoring).
  • Tow/lighting feature disabled where trailer-related functions are limited or shut down as a precaution.
  • Intermittent operation where trailer lights work sometimes and fail at other times, especially over bumps or during turns.
  • Multiple communication codes stored alongside U013A, indicating a broader network integrity concern.
  • Indicator anomalies such as unusual turn signal behavior related to trailer detection (varies by vehicle).

Common Causes

  • Trailer lighting module not powered due to an open fuse, relay issue, or interrupted power feed
  • Poor module ground path (loose fastener, corrosion, paint under ground point, damaged ground wire)
  • Open circuit, short-to-ground, or short-to-power in the network communication wiring between modules
  • Connector problems at the trailer lighting module or junctions (loose lock, backed-out pin, poor terminal tension, corrosion, moisture intrusion)
  • Damage to harness routing near the rear of the vehicle or hitch area (pinched, chafed, stretched wiring)
  • Network wiring faults at a splice or distribution point (high resistance, partial open, poor crimp)
  • Trailer lighting module internal fault (hardware) or software/logic fault that prevents communication
  • Low system voltage or unstable power supply affecting network integrity (battery, charging system, or high-load conditions)
  • Aftermarket wiring modifications or non-standard splices that disrupt power/ground or network continuity (varies by vehicle)

Diagnosis Steps

Tools that help: a scan tool capable of reading network DTCs and module status, a digital multimeter, and access to vehicle-specific wiring diagrams and connector views. If available, a breakout lead set and a current-capable test light can speed checks. Use the service information for connector pinouts, fuse locations, and any required module wake-up conditions.

  1. Confirm the concern and document codes: scan all modules for DTCs, record freeze-frame or event data if available, and note whether U013A is current or history. Also note any other network or battery/voltage-related DTCs that may explain a broader communication issue.
  2. Verify module presence on the scan tool: attempt to communicate directly with the trailer lighting module. If it will not respond, treat the issue as a power/ground/network access problem first rather than a lighting output fault.
  3. Check basics that can mimic a network fault: verify battery condition and charging system stability per service information, then clear codes and perform a short key cycle/road test to see if U013A resets immediately or only after certain conditions.
  4. Inspect related fuses and power feeds: using wiring diagrams, identify all fuses/relays that supply the trailer lighting module (constant and ignition-switched, if applicable). Confirm power is present at the fuse output side and that the fuse is not open. Do not assume a fuse is good by visual inspection alone.
  5. Inspect grounds and perform voltage-drop testing: locate the module ground point(s). With the circuit loaded (key on and as required by service info), perform voltage-drop testing across the ground path and across the power feed path to detect excessive resistance. Repair any poor connections before moving to network testing.
  6. Do a targeted visual inspection at the module and harness: disconnect the module connector(s) and check for backed-out terminals, bent pins, corrosion, moisture, damaged seals, and poor terminal tension. Inspect harness routing near the rear of the vehicle for chafing, pinch points, or repairs. Correct any connector/harness issues found.
  7. Check network integrity at the module connector: using service information pinouts, verify the network communication circuits are not shorted to ground or shorted to power with the module disconnected. Then verify continuity between the module connector and the next known good network point. Repair opens/shorts/high resistance as indicated.
  8. Wiggle test for intermittents: with the scan tool monitoring module communication status and network-related PIDs (where available), gently move the harness and connectors at the trailer lighting module, along the rear harness, and at splices/junctions. If communication drops in/out, isolate the section that triggers the fault and repair the wiring/terminals.
  9. Confirm wake-up and bus access conditions: some vehicles require specific ignition states, sleep/wake behavior, or network gateway conditions for the trailer lighting module to appear online. Follow service information to ensure the module is being commanded awake and that any related control modules are not setting a primary network fault that cascades into U013A.
  10. Re-test after repairs: clear DTCs, run a key cycle, and perform an operational check to confirm the trailer lighting module is communicating reliably. If the module now communicates, verify trailer lighting functions and monitor for code return during a longer drive cycle.
  11. If all external circuits test good, evaluate the module: only after confirming correct power feeds, ground integrity, and network wiring should you suspect the trailer lighting module itself. Follow service information for module replacement, setup, and any required configuration/programming steps (varies by vehicle).

Professional tip: Treat U013A as a communication problem first: verify power, ground, and network access at the trailer lighting module before chasing lighting output faults. Intermittent terminal tension issues can pass static continuity checks, so combine voltage-drop testing under load with live-data logging and a deliberate wiggle test to reliably reproduce the loss of communication.

Need network wiring diagrams and module connector views?

Communication stop and network faults require module connector pinouts, bus wiring routes, and power/ground diagrams. A repair manual helps you trace the exact circuit path before replacing any ECU.

Factory repair manual access for U013A

Check repair manual access

Possible Fixes & Repair Costs

Repair cost can vary widely because the underlying issue may be as simple as a connector problem or as involved as network diagnosis and module replacement. Parts, labor time, wiring access, and required programming (varies by vehicle) all affect the total.

  • Clean, reseat, and secure connectors at the trailer lighting module and any related junction points; repair terminal fit issues or corrosion found during inspection
  • Repair or replace damaged wiring in the module power, ground, or network communication circuits; restore proper routing and strain relief where harness damage is likely
  • Correct power/ground integrity problems by repairing weak grounds, loose fasteners, or high-resistance splices; confirm with voltage-drop testing after repair
  • Replace the trailer lighting module only after verifying power, ground, and network circuits are correct and communication remains lost
  • Perform required configuration, setup, or programming steps if a replacement module is installed (procedure varies by vehicle and scan tool capability)
  • Address related network faults (other U-codes) first if they indicate a broader communication or bus integrity issue

Can I Still Drive With U013A?

In many cases the vehicle may be drivable, but trailer lighting operation and related functions may be unreliable or disabled because communication with the trailer lighting module is lost. If you are towing, treat this as a safety-related concern and avoid driving until exterior and trailer lights are confirmed to function correctly. If any warnings indicate reduced power, braking, steering concerns, stalling, or a no-start condition, do not drive and diagnose the network/power issue immediately.

What Happens If You Ignore U013A?

Ignoring U013A can lead to persistent or intermittent loss of trailer lighting features and related warnings, increasing the risk of reduced visibility and signaling when towing. The fault may also mask a developing wiring, power/ground, or network integrity problem that can spread to other modules, creating additional communication codes and harder-to-trace intermittent faults over time.

Compare nearby lost trailer trouble codes with similar definitions, fault patterns, and diagnostic paths.

  • U0180 – Lost Communication With Automatic Lighting Control Module
  • U017D – Lost Communication With Control Module “M”
  • U017C – Lost Communication With Control Module “L”
  • U017B – Lost Communication With Control Module “K”
  • U017A – Lost Communication With Control Module “J”
  • U0179 – Lost Communication With Control Module “I”

Key Takeaways

  • U013A means the vehicle detected lost communication with the trailer lighting module, not a confirmed module failure.
  • Most successful repairs start with verifying module power/ground and checking connectors and harness condition.
  • Network issues can be intermittent; logging and wiggle testing help reproduce and pinpoint the dropout.
  • Replace the module only after circuit integrity tests confirm the wiring and network are healthy.
  • Towing safety may be impacted if trailer lighting functions are unavailable or inconsistent.

Vehicles Commonly Affected by U013A

  • Vehicles equipped with a dedicated trailer lighting control module
  • Vehicles with factory trailer tow wiring provisions and a multi-pin trailer connector
  • Vehicles that use a networked body electrical architecture to command exterior and trailer lighting
  • Vehicles frequently used for towing where harness strain, connector wear, or corrosion is more common
  • Vehicles with rear harness junctions, splice packs, or connectors exposed to moisture and road debris
  • Vehicles with recent electrical repairs, aftermarket wiring additions, or collision repairs near the rear harness area
  • Vehicles with intermittent battery voltage concerns or known low-voltage events affecting module communications

FAQ

Does U013A automatically mean the trailer lighting module is bad?

No. U013A indicates a communication loss was detected. The root cause is often wiring/connector issues, power/ground problems, or a network integrity fault. Confirm basics first (power, ground, and network wiring) before considering module replacement.

Will U013A affect the vehicle’s normal (non-trailer) exterior lights?

It depends on vehicle design. Some platforms keep normal exterior lighting independent, while others route certain trailer-related logic through the trailer lighting module. Verify lighting operation and review service information to understand which functions are shared.

Can a trailer or trailer plug cause U013A?

It can contribute indirectly. A damaged trailer connector, contamination, or wiring faults at the connector area can stress circuits or introduce intermittent electrical conditions that coincide with communication issues. Disconnect the trailer and inspect the connector and nearby harness before deeper network diagnostics.

Why does U013A come and go (intermittent)?

Intermittent U013A is commonly linked to loose connector locks, poor terminal tension, harness movement, or marginal power/ground connections. Network communication is sensitive to brief dropouts, so using live-data logging and performing a careful wiggle test can help locate the exact point of failure.

After repairs, do I need to clear the code or will it clear itself?

Many vehicles will require clearing codes after the fault is corrected, and some monitors may need a drive cycle to confirm the fix. Clear the DTC with a scan tool, then verify communication with the trailer lighting module and recheck for pending or returning network faults.

Always verify trailer lighting operation after repairs, especially if you tow, and confirm that U013A does not return during a road test with live-data monitoring where available.