B27A0

Code

Generic B — Body

Exterior Lighting Control Module Communication Fault

Brand: Generic

Type: B — Body

Views: UK: 5 EN: 11 RU: 5

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Page language: EN

Causes

Damaged, corroded, loose, or disconnected connector(s) at the exterior lighting control module
Blown fuse or failed power/ignition feed to the lighting control module
Poor or high-resistance ground at the module or shared ground point
CAN/LIN bus wiring shorted to power or ground, open circuit, or high resistance
Faulty exterior lighting control module
Other module on the network pulling the bus dominant or flooding the bus with errors

Symptoms

Exterior lights (headlamps, turn signals, DRLs, fog lamps) may be inoperative or behave intermittently
Dash warnings or MIL related to body lighting or communication
Loss of remote lighting functions (auto on/off, daytime running lights, adaptive features)
Other modules reporting communication errors or related DTCs
Intermittent or permanent lighting malfunctions that may disappear when vehicle restarted

What to check

Read stored and pending DTCs and freeze-frame data with a capable diagnostic scanner
Verify battery voltage at rest and with ignition ON (should be ~12–14.5 V)
Visually inspect connectors, wiring, and grommets at the exterior lighting module and along harness routing for damage or corrosion
Check fuses and relays related to lighting and module power supplies
Verify module ground(s) for corrosion, tightness, and continuity to chassis
Scan the vehicle bus for active messages and error counters; check for other modules reporting bus faults

Signal parameters

Module supply voltage: approx. 9–16 V depending on vehicle state (nominal ~12–14.5 V with engine off/on)
Module ground continuity: near 0 ohms to chassis (low resistance)
CAN bus idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (dominant state shows CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V) — differential ~2 V when dominant
Bus termination resistance: ~60 ohms measured end-to-end on a two-terminator CAN network
Message presence: expected periodic exterior-lighting messages on vehicle bus when ignition ON (use OEM service data for message frequency)

Diagnostic algorithm

Record all DTCs (including U-codes) and any freeze-frame data. Clear codes and see if B27A0 returns; note whether permanent or intermittent.
Verify battery voltage and perform a battery health check; low voltage can cause communication anomalies.
Inspect the lighting module connector(s) and harness for corrosion, bent pins, moisture, or physical damage. Repair or replace as required.
Check all relevant fuses and relays for the lighting system and module power feed; verify power pin(s) at the module with ignition ON.
Verify module ground(s): disconnect connector and measure continuity to chassis ground; clean and tighten ground connections if high resistance is found.
With a diagnostic scan tool, monitor CAN/LIN bus traffic. Confirm exterior lighting module messages are present and within expected frequency. Note any error frames or bus-off conditions.
Measure CAN_H and CAN_L voltages at the module connector with ignition ON and compare to expected idle/dominant values. Check for shorts to Vbatt or ground.
Measure bus termination resistance end-to-end (~60 ohms). An open or incorrect termination can cause communication faults.
Perform a wiggle test on wiring harness while monitoring bus messages and DTCs to reproduce intermittent faults (do this carefully to avoid damage).
If bus faults are traced to another module, isolate by disconnecting suspected node(s) one at a time while monitoring for code disappearance to identify the bad node.
If wiring, power, and grounds are good and no other module is causing the issue, consider reflashing or replacing the exterior lighting control module per OEM procedure. After replacement, ensure correct programming and configuration.
Clear codes and verify repair by checking for reappearance under the conditions the fault originally set.

Likely causes

Corroded or water-intruded connector at lighting module or harness (common in exterior modules)
Loose module ground or chassis ground corrosion
Damaged CAN bus wiring near front lighting area (accident damage, rodents, chafing)
Failed module due to internal electronics fault
Failed termination resistor(s) or incorrect bus termination after previous repair

Fault status

⚠️ Status

Communication fault detected: exterior lighting control module messages missing or corrupt on vehicle data bus; fault set when expected messages are not received or bus errors detected.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours