B1232

Code

MERCEDES-BENZ B — Body

Refrigerant pressure sensor (B12)

Brand: MERCEDES-BENZ

Type: B — Body

Views: UK: 12 EN: 30 RU: 10

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Page language: EN

Causes

Open or shorted sensor wiring or poor connector contact
Failed refrigerant pressure sensor (internal fault)
Low or excessively high system refrigerant pressure (actual pressure out of sensor range)
Blown fuse, faulty relay, or power/ground supply problem to sensor/module
Intermittent CAN/LIN communication fault (if sensor is on a bus)
Corrosion, moisture ingress, or mechanical damage at sensor or harness

Symptoms

A/C not cooling or reduced cooling performance
Compressor clutch does not engage or cycles incorrectly
A/C warning message or lamp on dash
Stored DTCs related to A/C control, pressure sensor, or communication
Intermittent A/C operation or fan behavior tied to A/C logic

What to check

Read stored and pending DTCs and freeze-frame data with a capable scan tool
Check live data: refrigerant pressure sensor raw value and calculated pressure
Visual inspection of sensor, connectors and wiring for damage, corrosion or water ingress
Check fuses and relays for the A/C system and sensor power circuits
Measure sensor supply voltage and ground at the connector with ignition on
Back-probe sensor signal while operating A/C to observe voltage changes

Signal parameters

Typical analog output: ~0.5–4.5 V (varies by sensor and model) where voltage corresponds to pressure range
Some Mercedes sensors use CAN/LIN messages instead of simple analog voltage — consult model-specific documentation
Expected pressure ranges depend on operating conditions: low-side ~1–3 bar (idle/cool), high-side can be 8–25+ bar (running); compare live values with manifold gauges
Reference supply commonly 5 V or sensor-specific supply — verify before testing

Diagnostic algorithm

Retrieve all A/C and body control module codes, note code freeze-frame and occurrence conditions.
Inspect the pressure sensor connector and harness for corrosion, bent pins, water intrusion, chafing, or damage; repair any visible faults.
With ignition ON, back-probe sensor connector: verify reference voltage (typically ~5 V), ground continuity, and sensor signal voltage. No reference or ground indicates wiring or fuse/relay fault.
Monitor live sensor data while operating A/C. A steady out-of-range value, no change when compressor cycles, or intermittent signal indicates sensor or wiring problem.
If sensor appears to be analog and wiring is intact, measure sensor resistance (if applicable) and compare to manufacturer values; otherwise substitute sensor signal by temporarily applying a known voltage for bench testing only if safe and documented.
Confirm actual refrigerant pressures with certified manifold gauges or vehicle service tool. If pressures are physically out of safe range, address leaks, overcharge, or compressor issues first.
Check associated fuses/relays and A/C compressor enable circuit (compressor clutch, relay, engine ECU command). Verify the control module is receiving the sensor data.
If wiring, power and control module communications are good and pressures are within range but the code persists, replace the pressure sensor and clear codes. Re-test operation and monitor for reoccurrence.
If a replaced sensor does not clear the issue, perform module-level diagnostics (check for software updates, module faults, or CAN/LIN bus errors) and consult manufacturer service information.

Likely causes

Damaged connector or pin corrosion at the pressure sensor
Sensor internal electronics failed (common after contamination or impact)
Wire chafing causing intermittent short to ground or battery
Loss of system refrigerant causing sensor to report low/out-of-range values
Faulty A/C control module or lost reference voltage to sensor

Fault status

⚠️ Status

Refrigerant pressure sensor (B12) signal implausible, out of range or missing; A/C control may disable compressor to protect system.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours

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B1232 — Refrigerant pressure sensor (B12)