B151912

Code

VOLVO B — Body

Panel / Defog Sensor Power Supply Circuit. Circuit Short To Battery

Brand: VOLVO

Type: B — Body

Views: UK: 12 EN: 18 RU: 9

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Page language: EN

Causes

Pinched or chafed wiring making contact with battery positive or another B+ conductor
Damaged or corroded connector creating unintended B+ connection
Internal short inside the panel/defog sensor (sensor body shorted to B+)
Incorrect or aftermarket wiring/repair that tied the circuit to constant battery power
Faulty relay or control module supplying permanent battery voltage to the circuit
Blown or incorrect fuse allowing abnormal power routing

Symptoms

DTC stored and MIL or HVAC warning lamp may be present
Defogger/defroster or instrument panel sensor readings incorrect or not working
Blown fuse(s) related to HVAC/defog sensor circuit
Loss of control over the panel/defog function (always on or always off)
Visible damage, melted insulation, or warm wiring in the dash area in severe cases

What to check

Visually inspect wiring and connectors for damage, melting, chafing, or corrosion around the panel/defog sensor and associated harness routing
Check fuses and related relays for the HVAC/defogger circuits
Backprobe the sensor power pin and measure voltage with ignition OFF and ON (and with HVAC/defogger commanded off and on)
Measure continuity between the supply circuit and battery positive with ignition OFF (should not be direct short to B+)
Disconnect the sensor connector and see if the code clears or voltage condition changes
Perform wiggle/pressure tests on the harness while monitoring voltage to reproduce an intermittent short

Signal parameters

Expected: With ignition OFF and system off — supply should be 0 V or high-impedance (no battery feed).
Expected: With ignition ON and system commanded on — supply normally sees battery voltage (~12–14.7 V).
Fault condition: Constant battery voltage (~12–14.7 V) present at the sensor power pin when it should be 0 V or switched (indicates short to B+).
If present, excessive current draw on the circuit indicates a short; measure for abnormal fuse current or use a clamp meter.

Diagnostic algorithm

Record freeze frame/conditions and any relevant symptoms before repair attempt.
Visual inspection: remove trim as needed and inspect wiring from sensor to fuse/relay/module for damage, pinched areas, or contact with other B+ wires or chassis harnesses.
Check fuses/relays: verify correct fuses and fusing; replace blown fuses only after locating short.
Backprobe sensor connector: with ignition off and system off measure voltage on the supply pin. If battery voltage present with key off, suspect short to constant B+.
Disconnect the sensor harness and measure voltage at the harness side and at the sensor side. If harness side still shows B+, the short is in wiring/connector or upstream. If harness side is OK and sensor side shows B+ only when connected, suspect sensor internal short.
Perform continuity test: with battery disconnected, measure resistance between the supply circuit and battery positive or other B+ circuits to find a low-resistance short path.
Isolate sections: disconnect connectors in-line (harness disconnects, fuse boxes) and re-test to locate the shorted segment.
If short is intermittent, perform wiggle/stress tests while monitoring voltage or use a powered fused jumper to localize (use caution and proper fusing).
Repair or replace damaged wiring, repair corroded connectors, or replace the sensor if internally shorted. Reassemble and clear codes, then verify repair by re-running tests and confirming no reappearance of the DTC.

Likely causes

Wiring harness short to a nearby constant positive feed (most likely)
Corroded/damaged sensor connector causing power feed to bypass switching
Failed panel/defog sensor with internal short (next most likely)
Faulty relay or module output stuck at B+ (less likely)

Fault status

⚠️ Status

Panel/Defog sensor power supply circuit shorted to battery (B+ present when circuit should be switched). Inspect wiring, connector, sensor, fuses, and control outputs.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-2.5 hours

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