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P0135 — O2 sensor heater circuit (Bank 1, Sensor 1)

P0135 — O2 sensor heater circuit (Bank 1, Sensor 1). A dead upstream O2 heater delays closed-loop mainly in cold weather—usually driveable once warm, but fix before mixture monitors and fuel economy suffer all winter.

OK to drive
OK to drive

Part of system: Electrical sensing & powertrain inputs

With P0135, prove the circuit before you condemn the part named in the scanner text. P0135 means the heater circuit for Bank 1 Sensor 1 (upstream oxygen sensor) failed to reach or maintain operating temperature. Without heater current the sensor stays cold after start, delaying closed-loop and skewing cold fueling—especially in winter. Measure heater resistance and power/ground at the sensor connector, check the heater fuse/relay, and inspect the harness near the exhaust manifold for melted insulation before replacing the sensor. Discriminate among open heater element inside Bank 1 Sensor 1, no power or ground to the O2 heater circuit, and blown fuse or failed heater relay/driver using voltage drop and connector integrity. Verify by measure heater resistance cold against OEM ohms specification, then verify battery voltage and ground at the sensor heater pins key-on; replace bank 1 Sensor 1 oxygen sensor with heater only when the wiring path checks out clean.

Vehicles often affected

Cold-climate cars where heater elements open after thermal cycling, or harnesses melt near exhaust manifolds.

Typical symptoms

  • Upstream O2 stays inactive for a long time after cold start
  • Closed-loop enable delayed with elevated cold-start fueling
  • Heater current PID near zero when commanded on
  • Blown heater fuse that refails after a shorted sensor is installed
  • Code sets more often on sub-zero mornings than in warm weather

Likely causes

  • Open heater element inside Bank 1 Sensor 1
  • No power or ground to the O2 heater circuit
  • Blown fuse or failed heater relay/driver
  • Melted or chafed heater wiring near the exhaust
  • Wrong sensor installed without a heater or with wrong pinout

Recommended checks

  1. 1

    Measure heater resistance cold against OEM ohms specification

  2. 2

    Verify battery voltage and ground at the sensor heater pins key-on

  3. 3

    Check the heater fuse and relay before replacing the sensor

  4. 4

    Repair melted harness sections with high-temp wiring practices

  5. 5

    Install the correct upstream sensor and confirm heater current PID

DIY vs professional

DIY-safe: check the heater fuse and sensor resistance. Pro: melted harness repairs near the manifold need high-temp wiring practices.

Parts to replace

See also