P0135 0xygen Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

What is Engine Code P0135?

  • P0135 Technical Definition: Oxygen Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
  • P0135 Meaning: Upstream Oxygen (O2) sensor heater circuit problem
  • Most common cause: Faulty Oxygen sensor, Faulty Oxygen sensor wiring or connector
  • Risks for the engine/driver: Low. You should get the car checked out as soon as possible
  • Emissions severity: HIGH. The car won’t pass emissions testing
  • Estimated repair time: 1 day
  • Estimated repair cost: $100+

The OBD2 engine code P0135 is stored in memory when the Electronic Control Unit (ECU) senses a problem in the oxygen sensor heater circuit corresponding to engine bank #1 (the bank of cylinders where cylinder #1 is located). The sensor 1 tag indicates the pre-catalytic converter O2 sensor.

The Oxygen Sensor (also known as O2 sensor) is in charge of reporting to the ECU the amount of residual oxygen present in the exhaust gases. The O2 information is in turn used by the ECU to determine if the fuel delivery was too high (rich condition) or too low (lean condition).

catalytic converter on exhaust system

In modern vehicles equipped with an A/F ratio sensor (still called O2 sensor by most technicians) the ECU deals with more information regarding fuel efficiency and hence adjusts fuel delivery with a higher degree of accuracy. The information coming from the O2 sensor is the main feedback used by the ECU to maintain the stoichiometric ratio of 14.7 parts of air for each part of fuel. That’s why the O2 sensor is so important for the emissions system.

Modern oxygen sensors come with an integrated heater circuit that is directly driven by the ECU. The heater helps the O2 sensor to reach its optimal operating temperature faster, especially during cold weather. Since the heater circuit is operated by the ECU, its constantly monitored as any other actuator.

The P0135 code is set when the ECU detects an abnormal electrical resistance in the heater circuit corresponding to the bank 1 sensor 1.

5 Common Symptoms When Code P0135 is Present

The most common symptoms of data trouble code P0135 are:

  1. Check Engine Light Flashing
  2. Diminished engine performance.
  3. Poor fuel economy
  4. Increased emissions
  5. Possible engine misfires

What Causes Error Code P0135

The most common causes of data trouble code P0135 are:

  • Damaged Oxygen sensor wiring (open, shorted, burnt) 
  • Faulty Oxygen sensor connector (loosely, corroded, disconnected or bent pins)
  • Faulty upstream oxygen sensor

How To Diagnose The Cause Of P0135

For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.

IMPORTANT: the diagnostic procedures listed below assume you only have the data trouble code P0135 present. If you also have any other code related to the ignition system, emissions system or fuel system then you should start diagnosing them first.

Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.

That said, let’s start the diagnostic process!

1. Preliminary steps

To discard a possible intermittent DTC condition, you’ll need to clear the ECU memory and complete a driving cycle.

  • Read data trouble codes and take note of them.
  • Clear data trouble codes memory.
  • Perform a driving cycle (at least 5-10 minutes).

In case the Check Engine Light stays off then you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.

2. Visual Inspection

> Oxygen Sensor wiring and connector: perform a meticulous visual inspection of the upstream O2 sensor wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug the sensor and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

Fix any wiring problem before continuing.

3. Scan Tool Tests

> Upstream Oxygen Sensor Test: start the engine to enter KOER mode. If your scan tool has a graphing ability turn it on. Graph engine RPM and O2 sensor output simultaneously. Accelerate the engine to 2000 RPM. You should see the voltage varying during acceleration and then stabilize. Decelerate to idle. You should see the opposite variation in the O2 output. Repeat the procedure several times. Some manufacturers include special tests for the O2 sensors. Follow on-screen instructions to complete those tests. Please notice that oxygen sensor output is completely different between wideband models and narrowband models, refer to the appropriate OEM literature to determine their expected behavior.

How To Repair Engine Error Code P0135

Depending on your diagnostic results, you may need to do the following:

  • Repair the oxygen sensor(s) wiring and/or connectors.
  • Repair the exhaust pipe
  • Replace the upstream oxygen sensor.
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