P0135 Code: O2 Sensor Heater Circuit (Bank 1 Sensor 1)

What the P0135 code means

An oxygen sensor only produces a useful signal above roughly 570°F (300°C). If the engine had to wait for the exhaust gas to heat it, that could take several minutes, and the whole time the engine would run in open loop, with a fixed fuel mixture and high emissions.

To avoid that, the sensor has a built-in heater that brings it up to working temperature in about 20 to 30 seconds. The computer powers that heater and measures the current flowing through it.

When that current is missing, or falls outside the expected range, the P0135 code is stored. The label bank 1, sensor 1 identifies the sensor ahead of the catalytic converter, on the side of the engine that has cylinder number one.

It helps to understand why this code makes you fail an inspection even when the engine runs fine. Without the heater, the sensor takes several minutes to reach temperature, and all that time the engine runs in open loop with a fixed mixture and high emissions. The oxygen sensor monitor never gets to run, and an inspection rejects a vehicle that has monitors that have not completed.

It also explains why fuel use goes up more on short trips than on the highway. On a ten minute drive the sensor never gets going, while on a long trip it eventually heats up from the exhaust gas alone and the system behaves normally.

P0135 code symptoms

• Check engine light on.
• Higher fuel consumption, especially on short trips.
• A slightly rough idle during the first few minutes after startup.
• The oxygen sensor monitor never reaches ready status, which keeps you from passing an emissions inspection.
• In some cases, no noticeable symptom while driving.
• Fuel economy that is clearly worse on short trips than on the highway.
• The engine takes several minutes to enter closed loop fuel control.

Likely causes of the P0135 code

1. Heating element in the sensor has gone open with age. This is the most common cause, especially above 93,000 miles (150,000 km).
2. Blown fuse in the heater circuit, which often affects several sensors at once.
3. Heater relay or circuit power missing.
4. Wiring damaged by exhaust heat or rubbed through against the body.
5. Corroded connector from its exposure under the vehicle.
6. Failed output driver in the computer. This is uncommon and is always checked last.
7. Heater relay with worn contacts, which cuts the current intermittently.
8. Moisture built up in the connector because of its position under the vehicle.
9. Universal replacement sensor with poorly made splices, which add resistance to the heater circuit.

How to diagnose the P0135 code step by step

1. Check for other heater codes. If P0135, P0141 and P0155 show up together, suspect the fuse or the shared power feed, not the sensors themselves.
2. Check the matching fuse in the engine compartment fuse box.
3. Measure the heater resistance between its two terminals with the connector unplugged. An infinite reading confirms an internal open circuit.
4. Verify that 12 volts reaches the heater power pin with the ignition on.
5. Check continuity and ground on the control wire running to the computer.
6. Inspect the harness along its whole length, especially where it passes close to the exhaust pipe.
7. Check how long the engine takes to enter closed loop. With a healthy heater it should not take more than a minute or two.
8. Measure the heater current, not just its resistance. A circuit with the right resistance can still have a weak power feed.
9. Check the heater relay by swapping it with an identical one from the fuse box.

How to fix the P0135 code

• Replace the oxygen sensor when the heater resistance reads open. Use the proper O2 sensor socket and put anti-seize on the threads.
• Replace the fuse and find out why it blew before you call the job finished.
• Repair the damaged section of wire with solder and heat-resistant sleeving, and route it away from the exhaust.
• Clean or replace the corroded connector.
• Make sure the replacement sensor is the correct one for your engine. Universal sensors that need splicing cause trouble often.
• Replace the heater relay if its contacts are worn, which often affects several sensors at once.
• Avoid universal sensors that need splices. The added resistance in the heater circuit brings the P0135 code back.
• Dry out the connector and protect it with dielectric grease, and make sure its seal sits firmly.

Technical reference data

• Typical cold heater resistance: 2 to 14 ohms, depending on the manufacturer.
• Heater supply: 12 volts, with the ground side controlled by the computer.
• Time to reach working temperature: 20 to 30 seconds after startup.
• Minimum sensor operating temperature: about 570°F (300°C).
• Bank 1 is the side of the engine that contains cylinder number one. Sensor 1 is the one ahead of the catalytic converter.
• Typical cold heater current: 0.8 to 2 amps, dropping as the element warms up.
• Time to enter closed loop with a healthy heater: 30 seconds to 2 minutes.
• A circuit with the correct resistance but a weak supply sets the same code, so measure current and not just ohms.

Brand-specific notes

On inline engines there is only a bank 1. On V engines, confirm in the service manual which side is bank 1 before you replace the sensor. Getting it wrong is an expensive and common mistake.

Universal sensors that come without a connector and require you to splice the wires are a recurring source of this code. The resistance the splices add is enough for the computer to set the P0135 code again.

Frequently asked questions about P0135