274B

What the code means

Code 274B points to a fault in the pump current calibration of the oxygen sensor (lambda sensor). The oxygen sensor measures the oxygen content in the exhaust, and the engine control unit (DME) uses its signal to correct the air-fuel mixture. In a wideband sensor with pump current, the pump current is proportional to the oxygen concentration. When this current is miscalibrated, the air-fuel ratio readings are skewed, which can hurt engine operation and emissions.

The code falls under the egr_dpf_scr category but is directly tied to mixture control. The fault can come from electrical problems (open circuit, short, poor contact in the sensor wiring) or from an internal failure of the sensor itself.

Symptoms

• Check Engine light: the check engine lamp comes on in the instrument cluster.
• Higher fuel consumption: bad sensor readings can make the control unit run the mixture rich or lean, which leads to excess fuel use.
• Increased emissions: a mixture that's off raises harmful exhaust emissions.

Diagnosis and repair

1. Check the oxygen sensor wiring: inspect the harness and connectors for damage, corrosion, opens, or shorts. Make sure the contacts are secure.
2. Replace the oxygen sensor: if the wiring is good and the code returns, fit a new sensor. Use original BMW parts or quality equivalents that meet the manufacturer's specs.
3. Reset adaptations and re-scan: after the repair, clear the fault code and take a test drive to confirm the error doesn't come back.