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DTC codes / BMW codes / P1135

P1135 · BMWO2 Sensor Heater Circuit Low Voltage Bank 1 Sensor 1

P1135 on a BMW means the engine control module has detected that the oxygen sensor heater circuit for Bank 1 Sensor 1 is drawing too little current or has a lower-than-expected voltage, indicating an open circuit, high resistance, or a failed heater element. The sensor is located before the catalytic converter on the bank that contains cylinder one, and its heater is responsible for bringing the sensor up to operating temperature quickly after a cold start.

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
  • Slightly rough idle or hesitation during the first minute or two after a cold start
  • A pending or stored P1135 code that returns quickly after being cleared
  • Occasional poor fuel economy if the fault persists and the sensor stays cold
  • In rare cases, a sulfur or rotten-egg smell from the exhaust if the sensor never reaches operating temperature

Common causes, most likely first

  1. Failed oxygen sensor heater element inside Bank 1 Sensor 1
  2. Corroded, loose, or damaged electrical connector at the oxygen sensor
  3. Broken or chafed wire in the heater circuit between the sensor and the engine control module
  4. Blown fuse or faulty relay that supplies power to the oxygen sensor heater circuit
  5. High resistance in the ground circuit for the oxygen sensor heater
  6. Engine control module internal driver failure for the heater circuit, though this is uncommon

How to diagnose and fix it

  1. Clear the code and verify it returns, then check for any related codes that might point to a shared circuit fault
  2. Inspect the oxygen sensor connector and wiring for corrosion, loose pins, or physical damage, and repair as needed
  3. Check the fuse and relay that feed the oxygen sensor heater circuit, replacing any blown fuse or faulty relay
  4. Measure the resistance of the heater element across the sensor-side terminals and compare to BMW specifications for that engine
  5. With the sensor disconnected and the key on, check for battery voltage at the heater power supply wire and good ground on the ground wire
  6. If voltage and ground are present but the heater resistance is out of spec, replace Bank 1 Sensor 1 with a quality OEM or equivalent sensor
  7. After repair, clear the code and perform a cold-start drive cycle to confirm the heater circuit monitors pass and the light stays off

What BMW techs check before replacing the sensor

  • A P1135 on BMW engines is frequently caused by a corroded connector or a chafed wire near the exhaust manifold rather than by the sensor itself, so always inspect the harness before condemning the part.
  • The heater circuit is powered through a fuse that is often shared with other emissions components, so a blown fuse can set P1135 alongside other codes and should be checked first.
  • On some BMW models, the oxygen sensor heater ground is shared with other sensors, and a poor ground can cause a low-voltage heater code even when the sensor heater tests good.
  • After replacing the sensor, the engine control module may need a short drive cycle to re-learn the heater characteristics, and clearing the code without completing the cycle can lead to a false return.
  • Using a generic universal oxygen sensor instead of the correct BMW-specific sensor can cause heater resistance mismatches that set P1135 again shortly after installation.

Can I keep driving with P1135?

You can usually continue driving with P1135 because the engine will still run and the main risk is slightly higher emissions during warm-up, but avoid long periods of idling or repeated short cold-start trips until the fault is repaired. If the check engine light begins flashing or you notice rough running, stop driving and have the vehicle diagnosed promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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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