Engine Code p0135 Meaning, Causes, Fixes (3 Steps)

Your check engine light illuminated on the dash again?

Don't worry, we've got you covered!

In this ProperMechanic.com guide, you'll learn:

  • What does the p0135 mean?
  • What caused the p0135 code (symptoms)
  • How to diagnose the code

And much more!

Engine Code p0135 Meaning, Causes, Fixes (3 Steps)

The OBD-II scanner shows a diagnostic trouble code p0135: O2 sensor 1 bank 1 heater circuit or heated oxygen sensor circuit malfunction.

On the typical vehicle exhaust system, there are two O2 sensors; bank 1 sensor 1 and bank 1 sensor 2. Sensor 1 comes before the catalytic converter, and bank 2 comes after the catalytic converter.

The reason the O2 sensor needs a heater is that it doesn't start reading the oxygen levels until it reaches 600 degrees F.

What does an O2 sensor do?

  • Reads the air/fuel ratio in the exhaust
  • Produces it's own voltage to send a signal to the engine's computer

Don't panic. We've got the knowledge you need to handle engine code p0135 means, causes, fixes, etc.

What Does The p0135 Code Mean?

The p0135 code means that the upstream heated oxygen sensor isn't getting the reading that it should. 

Often this means that it's time to replace the oxygen sensor that comes before the catalytic converter. 

But this isn't always the case. 

What Caused The p0135 code (Possible Symptoms)?

The cause of this code can be several different reasons. Some of the common causes of the p0135 code include:

  • failed oxygen sensor/heating element 
  • failed ECM(engine control module)/PCM
  • oxygen sensor heater circuit has an open circuit/is shorted
  • bad electrical connection(corroded connector)

The possible symptoms of the p0135 code are as follows:

  • check engine light illuminated
  • poor fuel economy
  • rough idling
  • failed engine starting

Read More >> What Are The Best Spark Plugs For Fuel Economy?

Did you know: Some other causes of a P0135 error code can include lead or antifreeze poisoning of the O2 sensor, oil deposits, and incorrect use of silicone gasket sealers.

How To Diagnose A p0135 Code?

This particular error code is saying that sensor number 1 reads the heating unit inside the O2 sensor is burned out and leaving an open circuit. Think of the heating unit as a light bulb; it eventually burns out and leaves an open circuit.

Here are the essential steps to diagnosing a p0135 code:

  1. Visual Inspection
  2. Removing The Sensor
  3. Bench Test 02 Sensor Heating Element
  4. Voltage Test
  5. Other possibilities

Step 1:  Visual Inspection

Locate the O2 sensor by either using your knowledge or by referring to the service manual of the specific vehicle. 

Visually inspect the wiring and electrical connector that goes to the upstream O2 sensor. 

Remember, this is an exhaust component. It is easy for the wiring to rub up against the exhaust pipe and burn, burn, burn. You have to remember this when reinstalling, also.

Step 2: Removing The Sensor 

Important: Let the vehicle cool down before trying to remove the sensor.

Removing the sensor is relatively easy; it just threads into the exhaust. 

First, unclip the O2 sensor electrical connection with your fingers or a pick tool, and separate the two sides of the connector. 

There is a special socket that can be extremely helpful in unscrewing the sensor from the exhaust. 

Tip: Alcohol in a spray bottle is your best friend when removing/installing electrical connections on cars. 

Remove the clips that are holding the O2 sensor cable away from the exhaust (don't forget reinstall after).

Read More >> What Are The Best Head Gasket Sealers?

Step 3: Bench Test O2 Sensor Heating Element

When the heater element burns out, it leaves an open circuit inside the O2 sensor, and the p0135 code causes the check engine light to illuminate.

You technically don't have to remove the sensor to test it as long as you have access to it's connector plug.

Continuity test: 

Set your multimeter to measure resistance in Ohms Ω.

Look for the two heater wires that are the same color-- these two are most commonly used for the heating element; you will know the sensor is still good if these two wires pass the continuity test.

Touch your test probes into the electrical pins of the wires that are the same color. If your multimeter beeps or shows no resistance, your O2 sensor is still good; you can then do a voltage test on the electrical connector that goes to the ECM.

Fun Fact: The operating temp of an O2 sensor is 600 degrees F

Step 4: Voltage Test

With the ignition on in the second position(without the engine running), test the electrical connector from the wiring harness that supplies the O2 sensor with power with a multimeter.

Set your multimeter to V, and insert two test leads into the corresponding pin holes that send voltage to the sensor(they should be the same color). 

You should get a reading of somewhere close to 12V. This means the battery is getting voltage.

Remember, if the heating element isn't getting battery voltage from, it won't heat up. Shown below is an excellent video on how to diagnose this exact repair. 

Step 5: Other Possibilities

In rare circumstances, the ECM isn't supplying the sensor with voltage, and you will need a replacement. 

It always helps to check all fuses in the fuse box when diagnosing engine problems. 

If you are still having problems, you may want to consult a professional automotive technician to give you a hand! 

Other issues could throw the code, like an engine coolant temperature sensor, or a bad catalytic converter

Read More >> What Does The P0420 Engine Code Mean?

How Do I Fix A p0135 Code?

Potential Fix 1: Bad Heating Element? Purchase A New Sensor

Most often, you are going to have to replace the O2 sensor. 

Whether you purchase online from sites like Amazon or your local auto parts store, use your VIN as a reference for finding the right part. 

Potential Fix 2: Sensor Not Getting Voltage? 

If the sensor isn't getting voltage, chances are either a fuse is blown or the cable has melted through on the exhaust. 

Visually inspect the wiring harness to verify it's still in good shape.

Check all associated fuses. 

The only other viable reason the sensor wouldn't be getting voltage would be a bad ECU.

Potential Fix 3: Replacing The Sensor 

After you have diagnosed the problem (probably a burned cable or needs a new sensor), replacing the sensor is the simple part.

The important part when installing exhaust sensors is making sure that you screw them into the threads before plugging them in; twisting the cable can cause a shortage further down the road. 

You may also want to put some anti-seize on the threads of the sensor before tightening.

The other important part about installing exhaust sensors is keeping the wiring away from the exhaust. Be sure to connect any retaining clips that protect the cable before starting the engine up.

Potential Fix 4: Reset The Check Engine Light

Reset the check engine light by either unplugging the battery or using the OBD-II scan tool.

About Your Mechanic

About Ryan Nichols

Ryan here! My pro mechanic career began as a technician for Mercedes-Benz. After two years of that great experience, I went out on my own in both the automotive and construction fields. I've since pulled my share of salvage yard parts and fixed more vehicles than I can count.

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