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DTC codes / BMW codes / P1156

P1156 · BMWO2 Sensor Heater Circuit Low Voltage Bank 2 Sensor 2

P1156 on a BMW means the engine control module has detected that the heater circuit inside the post-catalyst oxygen sensor on bank 2 (the side of the engine that does not contain cylinder 1) is drawing too little voltage, which usually indicates an open circuit, a failed heater element, or a wiring fault. The sensor's heater is responsible for bringing the sensor up to operating temperature quickly so the engine computer can trust its signal for catalyst monitoring and fuel trim corrections.

SystemPowertrain
Code typeBMW specific
SeverityLow
Fix difficultyIntermediate
AreaFuel and air metering

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms.
  • A slight increase in fuel consumption as the engine computer may default to a less efficient fuel strategy.
  • Occasional rough idle or hesitation during cold starts if the sensor stays cold too long.
  • A pending or stored code for catalyst efficiency or oxygen sensor readiness if the heater fault persists.
  • In rare cases, a noticeable sulfur or rotten egg smell from the exhaust if the sensor cannot report accurately.

Common causes, most likely first

  1. Failed heater element inside the bank 2 sensor 2 oxygen sensor itself, which is the most common cause on higher-mileage BMW engines.
  2. Corroded, chafed, or broken wiring in the sensor harness or connector, especially near the exhaust manifold where heat and vibration take a toll.
  3. Blown fuse that supplies power to the oxygen sensor heater circuit, often shared with other emissions-related components.
  4. Poor ground connection for the heater circuit at the engine block or chassis, leading to a voltage drop that the computer interprets as low voltage.
  5. Faulty relay in the oxygen sensor heater circuit, though this is less common on BMW models than on some other makes.
  6. Engine control module internal driver failure for the heater circuit, which is rare but possible after a short circuit or water intrusion.

How to diagnose and fix it

  1. Check the fuse that protects the oxygen sensor heater circuit and replace it if blown, then verify the new fuse does not blow immediately.
  2. Inspect the sensor connector and wiring for corrosion, chafing, or loose pins, and repair any damaged sections with proper automotive wire and heat-shrink tubing.
  3. Measure the resistance of the heater element inside the bank 2 sensor 2 oxygen sensor with a multimeter and compare it to the manufacturer's specification; an open circuit confirms a failed sensor.
  4. With the key on and the sensor disconnected, check for battery voltage at the heater power supply wire and a good ground on the ground wire to rule out wiring or relay faults.
  5. If voltage and ground are good but the heater resistance is out of spec, replace the bank 2 sensor 2 oxygen sensor with a quality direct-fit unit.
  6. Clear the fault codes and perform a road test that includes a cold start and enough driving time to allow the oxygen sensor heater monitor to run and set readiness.
  7. If the code returns after all checks pass, perform a voltage drop test on the ground circuit and inspect the engine control module connector for signs of water or corrosion.

Why a new sensor may not fix P1156

  • A common misdiagnosis is replacing the oxygen sensor when the real problem is a blown fuse or a corroded ground that starves the heater circuit of voltage, so always verify power and ground before condemning the sensor.
  • On many BMW engines, the bank 2 sensor 2 heater circuit shares a fuse with the secondary air pump or another emissions component, so a blown fuse may set multiple codes and the oxygen sensor code is just the first one noticed.
  • The heater element can fail intermittently when cold, so a resistance check on a warm engine may pass even though the sensor is faulty; let the car sit overnight and test the heater resistance when the sensor is at ambient temperature.
  • A low voltage code for the heater circuit does not always mean the sensor is dead; a poor connection at the engine control module can cause the same code, especially if the connector has been disturbed during recent service.
  • After replacing the sensor, the heater monitor may not run until the engine has completed a cold start and reached operating temperature, so do not assume the repair failed if the light stays off but readiness is not yet set.

Can I keep driving with P1156?

You can usually continue driving with P1156 because the engine will still run, but the oxygen sensor heater fault may cause slightly higher emissions and fuel consumption until it is repaired. Avoid long periods of idling or heavy load until the fault is fixed, and have the circuit checked soon to prevent catalyst damage from prolonged rich running.

Tools that help

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Guidance is general and written for BMW vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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