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Home / DTC / B1235 — Emissions sensor (B31)

B1235 — Emissions sensor (B31)

Detailed page for trouble code B1235.

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Code

B1235

MERCEDES-BENZ B — Body

Emissions sensor (B31)

Brand: MERCEDES-BENZ
Type: B — Body
AI status
Completed
ready
Completed 100%
Page language: EN

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

  1. Connect a diagnostic scanner, record B1235 and any other codes; note freeze-frame data and conditions when the code set.
  2. Obtain vehicle-specific wiring diagram and sensor pinout for emissions sensor B31 to identify reference, signal, and ground pins.
  3. Visually inspect sensor, connector, and wiring for damage, corrosion, water ingress, or heat exposure. Repair obvious damage before further testing.
  4. With connector plugged in, backprobe the reference, signal, and ground circuits. Verify reference voltage (approx. 5 V where applicable) and good ground.
  5. Observe signal voltage while engine is cold, at idle, and under load (or follow manufacturer test procedure). Compare to expected behavior in service manual.
  6. Perform continuity and resistance checks from sensor connector to control module pins with ignition OFF. Repair any open or high-resistance circuits.
  7. Wiggle harness and connectors while monitoring live data to detect intermittent faults. Check for corrosion pushing pins apart or poor contact.
  8. If wiring and connectors are good but signal is incorrect, replace the emissions sensor B31 with OEM or equivalent part and retest.
  9. 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.
  10. 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

Similar codes

B1234 B1250 P0130 P0131 P0141 P0420
856

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