Code
B1235
MERCEDES-BENZ
B — Body
Emissions sensor (B31)
Views:
UK: 12
EN: 17
RU: 13
AI status
Completed
Completed
100%
Causes
- Open or short in the sensor signal, reference, or ground circuit
- Corroded or loose connector/pins at the sensor or control module
- Failed emissions sensor (internal electronics or contamination)
- Poor ground or low reference voltage from vehicle wiring
- Aftermarket equipment or recent repair damaging harness
- Intermittent fault due to vibration or water ingress
Symptoms
- Malfunction Indicator Lamp (MIL) or warning message illuminated
- Possible reduced emissions system performance or limp mode in severe cases
- Failed emissions inspection or stored readiness faults
- Occasional or persistent diagnostic trouble code B1235
- Intermittent operation or irregular engine behavior if sensor influences engine control
What to check
- Read and record all stored DTCs and freeze-frame data with a capable scan tool
- Check for related engine/emissions codes (O2/NOx/EVAP) that may provide context
- Visually inspect sensor B31 connector and wiring for damage, corrosion, or pin push-out
- Wiggle test harness while monitoring live data to find intermittent faults
- Measure reference voltage and ground at the sensor connector (with harness connected)
- Backprobe signal wire and observe voltage/pattern with engine running or during test conditions
Signal parameters
- Sensor supply/reference typically driven from vehicle control module (commonly 5 V reference) — verify with manufacturer data
- Signal voltage expected to change with operating conditions (common ranges: 0.1–4.9 V for analog sensors; some sensors use 0–1 V or digital/serial outputs)
- Ground continuity: low resistance to chassis ground (
- Connector pin resistance and sensor element resistance vary by sensor type — consult factory service data for exact values
- Absence of signal or a constant rail voltage (0 V or full reference) indicates open/short condition
Diagnostic algorithm
- Connect a diagnostic scanner, record B1235 and any other codes; note freeze-frame data and conditions when the code set.
- Obtain vehicle-specific wiring diagram and sensor pinout for emissions sensor B31 to identify reference, signal, and ground pins.
- Visually inspect sensor, connector, and wiring for damage, corrosion, water ingress, or heat exposure. Repair obvious damage before further testing.
- With connector plugged in, backprobe the reference, signal, and ground circuits. Verify reference voltage (approx. 5 V where applicable) and good ground.
- Observe signal voltage while engine is cold, at idle, and under load (or follow manufacturer test procedure). Compare to expected behavior in service manual.
- Perform continuity and resistance checks from sensor connector to control module pins with ignition OFF. Repair any open or high-resistance circuits.
- Wiggle harness and connectors while monitoring live data to detect intermittent faults. Check for corrosion pushing pins apart or poor contact.
- If wiring and connectors are good but signal is incorrect, replace the emissions sensor B31 with OEM or equivalent part and retest.
- If new sensor does not resolve the issue, check control module input pin for short to Vb or ground and consider module bench testing or replacement per factory guidance.
- Clear codes, perform a test drive or required readiness cycles, and verify that B1235 does not return.
Likely causes
- Damaged wiring harness between sensor B31 and control module
- Corroded connector at the sensor
- Failed emissions sensor (most common)
- Broken ground or fused reference supply
- Moisture contamination of connector
Fault status
Status
Emissions sensor (B31) circuit fault detected — sensor signal is missing, out of range, or the circuit is open/shorted. Emissions monitoring may be compromised.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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