BMW 2742 — O2 Sensor Heater Resistance, Pre-Cat Bank 1

• Check engine light (MIL) illuminates after two consecutive drive cycles with the fault present.
• Slight influence on driveability during cold start warm-up — bank 1 runs open-loop longer than normal.
• Fuel economy may be marginally reduced on short trips.
• No breakdown risk.

1. Degraded oxygen sensor heater element. Over time and thermal cycling, the internal heating element develops increased resistance or partial internal shorts. This is the most frequent cause on sensors with significant mileage.
2. Corroded sensor connector pins. Moisture intrusion or heat cycling corrodes the connector pins, adding resistance to the heater circuit that the DME interprets as a sensor fault.
3. Damaged wiring harness. The harness section between the DME connector and the sensor runs near exhaust components. Heat damage, chafing against brackets, or rodent damage can cause open circuits (high resistance) or insulation breakdown (low resistance if shorted to another conductor).
4. DME software issue. ISTA data notes that DME software releases prior to version 07-06-510 had a known issue with this diagnostic. Software versions 07-09-521 and later include a fix. If the vehicle is running older DME firmware, a software update may resolve false fault triggers.

1. Check DME software version. Using ISTA+, verify the DME software level. If the installed version predates 07-06-510, program the DME to the latest available software before proceeding with physical diagnosis — the fault may be a false positive from the known software issue.
2. Locate and inspect the bank 1 pre-cat O2 sensor connector. On V8 engines, this is on the driver-side exhaust manifold. Disconnect the connector and inspect for corrosion, heat damage, moisture, or deformed pins.
3. Measure heater resistance at the sensor. With the sensor disconnected, measure resistance across the heater pins. Expected range: approximately 6–14.5 ohms at room temperature. Record the exact reading.
   • Below 6 ohms → internal short in heater element → replace sensor.
   • Above 14.5 ohms → open or high-resistance heater element → proceed to wiring check before condemning the sensor.
   • Within range → fault may be intermittent or temperature-dependent. Proceed to step 5.
4. If resistance is high: check wiring continuity. Measure resistance from the DME harness connector (heater pins) to the sensor connector pins. Compare against the reading at the sensor. A difference of more than 1–2 ohms indicates a wiring problem — inspect the harness for damage, focusing on the section near the exhaust.
5. If resistance is within range at room temp: The heater element may fail under thermal load. With the engine running and the sensor connected, use ISTA+/INPA to monitor the real-time heater resistance calculation. If it drifts outside the threshold window as the engine bay heats up, the sensor is failing under temperature and should be replaced.
6. Check heater supply voltage and ground as described for fault 2741 to rule out power delivery issues.

If DME software is outdated (pre-07-06-510), update the DME software first and clear the fault. Monitor over 2–3 drive cycles.

If the fault persists or software is already current, replace the bank 1 pre-catalyst oxygen sensor. Use an OEM or OEM-equivalent sensor. Consult BMW ETK or RealOEM for the correct part number for your engine and production date.

After replacement, clear the fault, perform a cold-start drive cycle, and re-scan to confirm the code does not return.