P0135 – O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

What does P0135 mean?

The heater circuit in the upstream oxygen sensor (Bank 1, Sensor 1) is not working correctly. Oxygen sensors need to reach roughly 600°F to read accurately, and the built-in heater brings them up to temperature quickly after a cold start. Without it, the sensor takes too long to become useful.

What causes P0135?

Listed from most to least common:

1. 1Failed oxygen sensor heater element (most common)
2. 2Blown fuse for the O2 sensor heater circuit
3. 3Damaged wiring or connector at the sensor — often from exhaust heat
4. 4Corroded connector
5. 5Relay fault in the heater circuit
6. 6Rarely, an ECM fault

Code breakdown

Common symptoms

• Check engine light on
• Slightly worse fuel economy
• Usually no noticeable change in drivability
• Failed emissions test

How to diagnose

1. 1Check the fuse for the O2 heater circuit first — it is the cheapest possible cause.
2. 2Inspect the sensor connector and wiring for heat damage, melting, or corrosion.
3. 3Measure the heater element resistance across the sensor's heater terminals and compare against specification. An open circuit confirms a failed heater.
4. 4Verify battery voltage is reaching the heater circuit with the key on.
5. 5If voltage is present and resistance is out of spec, replace the sensor.

The engine runs in open loop longer after startup, hurting fuel economy and emissions. Not urgent, but it will fail emissions testing.

Common fixes and estimated costs

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

Use a proper oxygen sensor socket for removal, and apply anti-seize sparingly to the threads only — never on the sensor tip. Sensors that have been in place a long time can seize; heating the surrounding metal slightly can help.

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