B1798

Code

Other B — Body

Lamp Turn Signal Front Output Circuit Failure

Brand: Other

Type: B — Body

Views: UK: 25 EN: 37 RU: 31

AI status

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Page language: EN

Causes

Blown or burned-out front turn signal bulb
Corroded or loose lamp socket or connector
Open or shorted wiring between BCM and front lamp
Poor ground at lamp or chassis ground point
Faulty BCM/lighting module output transistor or driver
Aftermarket LED conversion without proper load/resistor

Symptoms

Front turn signal (left or right) not illuminating
Turn signal indicator on dash may not illuminate or may flash rapidly
One or both front parking/turn lamp functions not working
DTC stored in BCM and possible reduced/disabled lamp function
Intermittent lamp operation or flicker
Other lighting functions may operate normally

What to check

Scan vehicle and read freeze-frame and lamp-related codes; note which side (left/right) is affected
Visually inspect front lamp, bulb, socket and connector for damage, corrosion, or water
Check fuse(s) and related relays for the lighting circuits
Command the front turn lamp ON/OFF with a scan tool and observe lamp behavior
Measure voltage at the lamp connector with turn signal commanded (should be ~battery voltage when ON)
Measure continuity/resistance from BCM output pin to lamp socket and to ground

Signal parameters

Nominal supply voltage: ~11–14.5 V at lamp connector when output is active
Inactive output: near 0 V (or open) depending on BCM design
Pulse rate: ~1–2 Hz (60–120 flashes per minute) for turn signals; rate may vary by vehicle
Expected current draw (typical halogen bulb): ~0.3–2.5 A depending on bulb wattage
Expected current draw (LED bulbs): tens to a few hundred mA; may be too low for some BCM diagnostics
Ground continuity to chassis: low ohms (typically

Diagnostic algorithm

Connect a scan tool, read DTC B1798 and any related codes; record freeze-frame and clear codes, then re-run to verify persistence.
Identify affected side (front left or right) and perform a visual inspection of lamp assembly, bulb, socket, and wiring for corrosion, damage, or moisture.
With ignition on, command the affected front turn signal ON via scan tool while an assistant observes the lamp. Note if lamp illuminates, flickers, or remains off.
Measure voltage at the lamp connector with the output commanded ON. If full battery voltage is present at the connector and lamp still fails, suspect bulb/socket or ground issue.
If no voltage at connector, backprobe the BCM output pin (with wiring diagram) and check for voltage at the module. If output is present at BCM but not at lamp, locate and repair wiring open or connector fault.
Check resistance/continuity between BCM output pin and lamp socket. Repair any open or high-resistance wiring.
Check for shorts to ground or battery by measuring resistance from output circuit to ground and battery when lamp disconnected. Repair short circuits as needed.
Inspect and test ground continuity from lamp ground to chassis. Repair poor ground connections.
If wiring and lamp are confirmed good but BCM output does not activate or shows abnormal current, consider BCM/lighting module internal failure. Verify with manufacturer diagnostics before module replacement.
If aftermarket LEDs are installed, reinstall original bulbs or add proper load resistors and re-test, or reprogram BCM if vehicle supports LED loads.
After repair, clear codes and confirm turn signal function through multiple cycles; road test to ensure fault does not recur.

Likely causes

Faulty bulb or poor bulb-to-socket contact
Damaged wiring or connector corrosion near the lamp
Failed BCM output driver for the affected lamp
Aftermarket LED bulb lowering circuit load and triggering fault

Fault status

⚠️ Status

The BCM detected an abnormal condition on the front turn signal lamp output circuit (open, short, low/high current, or intermittent). A fault is stored and related lamp function may be disabled or unreliable.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2 hours