B1258

Code

ALFA ROMEO B — Body

Solar radiation sensor circuit

Brand: ALFA ROMEO

Type: B — Body

Views: UK: 7 EN: 12 RU: 7

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Page language: EN

Damaged or disconnected solar radiation sensor
Broken, chafed or corroded wiring or connector (open/short to ground or battery)
Poor ground or missing reference voltage at sensor
Blown fuse or poor power supply to the climate/BCM circuit
Water ingress or physical damage to sensor (UV exposure, cracked lens)
Faulty climate control module/BCM or software issue

DTC B1258 stored and MIL (if applicable) may be lit
Automatic climate control behaves incorrectly (overcooling/overheating, slow to react)
Sunlight-based HVAC adjustments (solar compensation) not working
Occasional or constant fault message related to sensor in instrument cluster or infotainment
Inconsistent cabin temperature control under changing sunlight

Read freeze frame and live data with a diagnostic scan tool; confirm B1258 and note related codes
Locate solar sensor (usually on top of dash near windshield) and visually inspect for damage, contamination or shading
Inspect connector pins for corrosion, bent pins, or poor mating; check for water entry
Check relevant fuses and relays for climate/BCM power
Back-probe sensor connector and check for reference voltage (typically 5 V) and ground
Measure sensor output voltage while varying light level (cover/uncover) to confirm response

Reference supply: typically ~5.0 V (check vehicle wiring diagram for exact value) ±0.2–0.5 V
Ground: approx. 0 V with low resistance to chassis ground
Signal output: varies with illumination — approximate dark =
Typical sensor response: voltage should change smoothly with changing light; abrupt/no change indicates fault
If available, live-data lux or percent values should increase with sunlight and decrease in shade

Connect a diagnostic scan tool, read stored/freeze-frame data and note any related codes (HVAC, BCM).
Check for recent software updates or technical service bulletins related to solar sensor and climate control.
Visually inspect sensor, lens, and harness; remove debris or clean lens if dirty. Ensure sensor is not shaded by accessories.
Verify fuse(s) and power supply to the climate/BCM circuits mentioned in wiring diagram.
Back-probe the sensor connector: verify reference voltage (≈5 V) and ground with ignition ON. If reference missing, trace back to BCM/fuse.
With a multimeter, measure sensor output voltage in low light and in bright light (cover/uncover). Confirm voltage changes and falls within expected range.
If signal is open or shorted: perform continuity/resistance checks from sensor connector to control module to locate wiring damage.
Wiggle test wiring while monitoring live data to find intermittent faults caused by broken wires or poor connectors.
If wiring and connector are good but signal is out of range or no response, replace the solar sensor and retest.
Clear codes, re-check live data under different lighting conditions, and verify code does not return. If it does, proceed to inspect/replace HVAC control module or investigate for module software issues.

Connector pushed out, pins bent, or corrosion causing intermittent/open circuit
Signal wire shorted to ground or battery (voltage too low or too high)
Sensor has failed (internal electronics degraded)
Harness rubbed through at dash top or A-pillar area
Module is not receiving or not interpreting sensor reference/signal due to internal fault or software

⚠️ Status

Solar radiation sensor circuit — open/short/implausible signal detected by climate control/BCM. Check sensor, wiring, power and ground.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-1.5 hours

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