What does P0135 mean?

The heater in the upstream oxygen sensor is not working as expected.

Why does the oxygen sensor have a heater?

So it reaches working temperature quickly and the ECU can start precise fuel control sooner.

Emission control

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

The heater inside the upstream oxygen sensor on bank 1 is not working correctly. Without the heater the sensor takes too long to warm up, so the ECU stays in open loop longer.

Severity: Medium

Common causes

Failed oxygen sensor heater element
Blown heater fuse or relay
Open or shorted wiring to the heater
Corroded connector
Faulty engine control module driver (rare)

Symptoms

Higher fuel consumption on cold starts
Check engine light
Rough idle when cold
Failed emissions test

How to check

Check the heater fuse and relay
Measure the heater resistance (usually a few ohms) at the sensor connector
Check the heater supply voltage and control signal
Replace the sensor if the heater is open

Can you keep driving?

Drivable. Fuel economy and emissions suffer, so repair it at a convenient time.

Frequently asked questions

What does P0135 mean?: The heater in the upstream oxygen sensor is not working as expected.
Why does the oxygen sensor have a heater?: So it reaches working temperature quickly and the ECU can start precise fuel control sooner.

General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.