P0135: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)

P0135 means the internal heater element of your upstream (Bank 1, Sensor 1) oxygen sensor has failed, so the sensor warms up too slowly and the engine delays entering closed-loop fuel control. The most common cause is simple heater wear inside the sensor. Because this pre-catalyst sensor sets the air-fuel mixture, you mainly notice a check engine light and worse cold-start fuel economy. A replacement upstream O2 sensor costs 50 to 200 EUR including labour.

P0135 is a emissions-related diagnostic trouble code classified as low severity. When your vehicle's ECU detects this condition, it stores P0135 and illuminates the check engine light.

If your vehicle has triggered P0135, you may notice one or more of these symptoms:

• Check engine light illuminated
• Slightly worse fuel economy during cold starts and short trips
• Engine stays in open-loop (rich) mode longer than normal after starting
• Higher cold-start emissions, possible failed emissions test
• Possible rough or slightly unstable idle during the warm-up phase
• No noticeable power loss once the engine is fully warm

Here are the most common causes of P0135, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Safe to drive but more impactful than P0141. The upstream O2 sensor directly controls the fuel mixture, so the engine runs in open-loop (rich) mode until exhaust heat warms the sensor. Fuel economy and emissions are noticeably worse during cold starts.

Follow these steps to pinpoint the root cause of P0135:

1. Read all stored codes and note whether other heater codes (P0141, P0155, P0161) are present. Multiple heater codes together usually point to a shared fuse, relay, or wiring fault rather than one sensor
2. Locate and check the O2 sensor heater fuse in the fuse box. Replace it if blown and recheck before condemning the sensor
3. Unplug the Bank 1 Sensor 1 connector and measure heater resistance across the two heater pins. A healthy heater reads roughly 2-15 ohms. An open circuit (infinite reading) means the element has burned out, under 1 ohm suggests a short
4. Inspect the connector and pins for corrosion, melting, bent terminals, or water intrusion, and clean or repair as needed
5. Back-probe the connector with the key on and check for battery voltage on the heater supply wire. No voltage points to a fuse, relay, or wiring fault upstream of the sensor
6. Inspect the sensor wiring along its full run for chafing or melting against the exhaust manifold and heat shields, then clear the code and confirm it does not return

Repair costs for P0135 vary depending on the root cause and your vehicle. Here are typical estimates:

P0135 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0135 or indicate related issues: