P0135 moderate

O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)

What does P0135 mean?

The P0135 code means the vehicle's onboard diagnostic system has detected: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1). This is a moderate severity code.

Common Symptoms

  • Check Engine Light on
  • Extended open-loop operation after cold start (rich running while warm-up)
  • Poor cold-start fuel economy

Probable Causes (Ranked by Likelihood)

  • Failed O2 sensor heater element Very High
  • Blown fuse for O2 sensor heater circuit Moderate
  • Open or shorted wiring to heater circuit Moderate

Step-by-Step Diagnostic Procedure

  1. Check the O2 sensor heater fuse in the fuse box.

  2. Test heater element resistance at the sensor connector: typical spec is 3-15 ohms. Open circuit means the heater is burned out.

  3. With the key on engine running, verify 12V supply to the heater circuit at the connector.

  4. Replace the O2 sensor if the heater element has failed.

Frequently Asked Questions

Why does the O2 sensor have a heater?

The heater brings the sensor to operating temperature (approximately 600 degrees F) quickly after a cold start so the engine can enter closed-loop fuel control within 30-60 seconds instead of waiting for exhaust heat alone, which could take several minutes.

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