P0135 – Oxygen sensor heater circuit malfunction (bank 1, sensor 1)

marzo 06, 2025

P0135 – Oxygen sensor heater circuit malfunction (bank 1, sensor 1)

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Technical definition

O2 sensor heater circuit bank 1 sensor 1 malfunction

What does P0135 mean?

The P0135 diagnostic trouble code indicates a problem with the heater circuit in the upstream oxygen (O₂) sensor, which is located before the catalytic converter on bank 1.

When you start your car, the engine control module (ECM) runs a self-check on the oxygen sensor’s heater circuit. This built-in heater helps the sensor reach its operating temperature quickly, ensuring accurate emissions readings from the start.

If the ECM detects an issue, it triggers the P0135 code.

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What is bank 1?

Engines with V-shaped, W-shaped, flat, and some inline-6 configurations have their exhaust system split into banks. Bank 1 is the side containing the first cylinder – usually the closest one to the front of the engine, near the drive belt.

In engines with only one bank (like most inline engines), everything is simply referred to as Bank 1.

The image shows a longitudinal engine with Bank 1 and Bank 2, each having upstream and downstream oxygen sensors before and after the catalytic converters.

The image shows a longitudinal engine with Bank 1 on the right and Bank 2 on the left, each featuring upstream (Sensor 1) and downstream (Sensor 2) oxygen sensors placed before and after the catalytic converters.

Engine banks. Source: densoautoparts.com

What does an oxygen sensor do?

Oxygen sensors measure the oxygen levels in the exhaust gases. A vehicle can have up to five of them.

This specific sensor, located upstream of the catalytic converter, helps the vehicle's computer fine-tune the air-fuel mixture for optimal performance. By analyzing oxygen levels in the exhaust, the ECM adjusts fuel delivery in real time, keeping combustion clean and emissions low.

Why do oxygen sensors need a heater?

O₂ sensors only work at high temperatures – from 316°C (600°F).

Older, single-wire sensors rely on exhaust heat to warm up, which takes time. Heated sensors, like the one in your car, use an internal heater to reach the right temperature almost immediately. This ensures accurate readings from the moment you start the engine.

How serious is P0135?

A faulty oxygen sensor heater makes it harder for your car to regulate the air-fuel ratio, especially during cold starts. Without a working heater, the engine may run rich at first, leading to lower fuel efficiency, increased emissions, and potential damage to the catalytic converter.

This issue could also cause your vehicle to fail an emissions test. Ignoring this problem for too long can result in more costly damage.

Can I still drive with P0135?

Yes, you can keep driving, but it’s best to fix the problem soon. A malfunctioning heater circuit can increase fuel consumption and emissions.

Common symptoms of P0135

Watch for these warning signs:

Check engine light

Decreased fuel efficiency

Misfires

Rough idle

Common causes of P0135

Most often, the P0135 error code appears due to electrical issues:

Faulty oxygen sensor

Damaged or loose wiring/connections around the oxygen sensor

Short or open circuit in the wiring

Corrosion on the oxygen sensor or electrical connector

Excessive current draw from the oxygen sensor

Blown fuse

How to troubleshoot P0135

Clear the codes and test drive

Use your OBD code reader to clear the P0135 fault code.

If the fault code immediately reappears: The issue is still active. Skip the test drive and move to the next steps.

If the codes clear successfully: Drive your car for about 15 minutes to fully warm up the engine and observe its behavior.

After the test drive, scan again to see if the code returns. If it does, the problem needs further investigation.

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Examine the oxygen sensor circuit

For this test, you'll need a digital multimeter.

Locate the sensor. The upstream oxygen sensor (bank 1, sensor 1) is in the exhaust pipe before the catalytic converter. If unsure, check your vehicle’s service manual for the exact location.
Inspect wiring and connections. Look for damaged, frayed, or disconnected wires. Check the sensor’s connector for corrosion or loose pins.
Check the fuse. Some O₂ sensor heater circuits have a fuse. If yours does, check your owner’s manual and replace it if needed.
Test the heater element. Disconnect the sensor and use a multimeter to measure resistance across the heater terminals (two wires of the same color). A normal range is 5-20 ohms. If the reading is outside this range, the sensor may be faulty.

Replace oxygen sensor

If the wiring and connections look good, but the code persists, it's time for a new O₂ sensor.

Unplug the old sensor. Find the sensor’s electrical connector and disconnect it.
Remove the old sensor. Use a wrench to loosen and unscrew it. If it’s stuck, apply penetrating oil and wait a few minutes.
Install the new sensor. Thread it in by hand first to avoid cross-threading, then tighten it with a wrench (but don’t overtighten).
Reconnect the wiring. Plug the new sensor’s electrical connector back in, ensuring a secure fit.
Clear the code and restart the car. Use an OBD II scanner to reset the check engine light. Start the car and let it run for a few minutes. Then, take a test drive and scan again to confirm the issue is resolved.

Oxygen sensor

FAQ

How much does it cost to fix code P0135?

Fixing P0135 is usually an inexpensive repair. Depending on your vehicle, a new oxygen sensor costs between $20 and $200. Labor can add another $50 to $200 if you go to a mechanic.

Can an exhaust leak cause P0135?

No. An exhaust leak won’t cause P0135 because this code is related to the oxygen sensor’s heater circuit, not the exhaust flow. However, an exhaust leak can lead to incorrect oxygen sensor readings, which may trigger other codes related to fuel mixture or efficiency.