BMW 2743 — O2 Sensor Heater Resistance, Pre-Cat Bank 2

Severity: Informational
Module: DME
OBD-II Code: P0155

Description

Fault code 2743 is set by the DME when the measured resistance of the bank 2 pre-catalyst oxygen sensor heater element falls outside the acceptable range. This is the bank 2 counterpart to fault code 2742 — the diagnostic logic, thresholds, and failure modes are identical, but the monitored sensor is on the passenger-side exhaust bank.

This code applies to BMW engines with split exhaust manifolds — both inline-6 (where bank 2 is cylinders 4–6) and V-configuration V8/V12 engines (where bank 2 is the passenger-side cylinder bank).

The DME continuously calculates the oxygen sensor heater resistance from measured current and voltage. After approximately 100 valid calculations (~1.5 seconds), if the computed resistance falls below ~6 ohms or exceeds ~14.5 ohms, this fault is stored. The diagnostic logic is identical to fault code 2742 but monitors the bank 2 (passenger side) pre-catalyst sensor.

Safety Warnings

This is a DME fault. Disconnect the battery negative terminal before working on sensor connectors to avoid short circuits. A non-functional O2 sensor heater causes extended open-loop fueling during warm-up, which increases catalyst thermal loading over time.

2743 - P0155: O2 Sensor Heater Resistance, Pre-Cat Bank 2 - Heater resistance out of range

Symptoms

Check engine light (MIL) illuminates after two consecutive drive cycles with the fault present.
Slight influence on driveability during cold start warm-up — bank 2 runs open-loop longer than normal.
Fuel economy may be marginally reduced on short trips.
No breakdown risk.

Common Causes

Degraded oxygen sensor heater element. Thermal cycling over high mileage causes the heater element resistance to drift outside the acceptable window. Most frequent cause.
Corroded sensor connector pins. The bank 2 sensor connector on the passenger side can be more exposed to road spray on some chassis, making corrosion slightly more common than bank 1 on certain models.
Damaged wiring harness. Heat damage or chafing near exhaust components on the passenger side.
DME software issue. As with 2742, DME software prior to version 07-06-510 has a known issue with this diagnostic. Versions 07-09-521 and later include the fix.

Diagnosis Steps

Check DME software version. Using ISTA+, verify the installed DME software level. If it predates 07-06-510, update to the latest software before proceeding — the fault may be a false positive.
Locate and inspect the bank 2 pre-cat O2 sensor connector. On V8 engines, this is on the passenger-side exhaust manifold. Disconnect and inspect for corrosion, heat damage, moisture, or deformed pins.
Measure heater resistance at the sensor. Disconnect the sensor and measure across the heater pins. Expected: approximately 6–14.5 ohms at room temperature.
- Below 6 ohms → internal short → replace sensor.
- Above 14.5 ohms → open or degraded element → check wiring before condemning sensor.
- Within range → may be intermittent; proceed to step 5.
If resistance is high: check wiring continuity. Measure end-to-end from the DME harness connector to the sensor connector. A difference greater than 1–2 ohms from the sensor-side reading indicates harness damage.
If resistance is within range at room temp: Monitor the live heater resistance value with ISTA+/INPA while the engine is running. If the value drifts out of spec as temperature increases, the sensor is failing under thermal load.
Check heater supply voltage and ground path to rule out power delivery issues (see fault 2741 diagnosis steps 4–5).

Resolution

If DME software is outdated (pre-07-06-510), update first and monitor.

If the fault persists or software is current, replace the bank 2 pre-catalyst oxygen sensor with an OEM or OEM-equivalent part. Consult BMW ETK or RealOEM for the correct part number.

After replacement, clear the fault code, complete a cold-start drive cycle (idle 2–3 minutes, then 10–15 minutes moderate driving), and re-scan to confirm the fault does not return.

If both 2742 and 2743 are stored simultaneously, inspect the shared power supply circuit before replacing both sensors — a common supply wire fault can affect both banks.

Module Reference: DME