Ford Fusion P0135 Trouble Code – Test your 02 Heater Circuit

The OBDII Ford Fusion P0135 trouble code means that there’s an issue with the heater circuit in your primary oxygen sensor. Your primary oxygen sensor is also known as the upstream 02 sensor. This 02 sensor is mounted directly after your headers and before your catalytic converter. Your primary 02 sensor is designed to read the exhaust to determine how efficiently your engine is performing. Based on these signals your engine computer will correct and adjust the fuel trims accordingly.

When the powered section of your primary 02 sensor or the heater circuit has issues, the engine computer will store the Ford Fusion P0135 check engine light. When the P0135 trouble code rears it’s ugly head, your Fusion engine will run roughly.

Ford Fusion P0135 Trouble Code 1

Today I’ll be showing you how to check the heater circuit in your primary oxygen sensor in a 2.5 liter equipped 2014 Ford Fusion. There are several symptoms of a Ford Fusion P0135 OBDII check engine code.

Symptoms of a Ford Fusion P0135 DTC trouble code

There are several issues that can arise when your primary 02 sensor heater circuit goes out. Because the heater circuit powers the front upstream oxygen sensor, the P0135 code will prevent your engine computer from managing the fuel and spark events of your engine efficiently. Here are some of the symptoms you will see with a P0135 check engine code.

  • Poor fuel economy
  • Black smoke from exhaust
  • Harsh shifting from automatic transmission
  • Check engine light ON

The check engine light being on in your 2014 Ford Fusion can stop you from passing your emissions test. If you have a problem with your primary 02 sensor heater circuit, use this guide to test your upstream oxygen sensor.

Ford Fusion P0135 Trouble Code

If you’ve used a OBDII scan tool to retrieve your Ford Fusion P0135 trouble code, you will need to use a digital multimeter to test the 02 heater circuit. You will be checking the performance of your primary 02 sensor using this tool. If you are not sure what a multimeter is, check out this handy guide here. The exhaust on your 2.5 liter Ford engine is facing towards the rear of the car, or your firewall so it’s best to test the heater circuit when the engine is cold.

Ford Fusion P0135 Trouble Code 2

The primary oxygen sensor is screwed into your exhaust just after the collector, shown above. Trace back the wiring from the upstream oxygen sensor and you’ll see the four pin weatherproof connector on top of the valve cover.

Ford Fusion P0135 Trouble Code 3

To free your primary 02 sensor connector, pull up gently on the connector and depress the locking pin. Unplug the upstream Fusion oxygen sensor and remember you will be testing the ENGINE harness side of the 02 sensor.

Ford Fusion P0135 Trouble Code 4

Now that your Ford Fusion primary 02 sensor is unplugged you can test the engine harness to make sure that you are receiving power and ground at the sensor connector. The first order of business is to check your primary 02 sensor for power, which is a vital part of your heater circuit.

Testing your Ford Fusion primary 02 sensor for power

Unfortunately the engine harness side of your Ford Fusion primary oxygen sensor is made up of male terminals. While this isn’t a big deal, you can cause damage to your ECU if you accidentally touch or jump two of the wrong pins. If you jump or touch two pins at once, you run the risk of damaging your Ford Fusion ECU, so take care to test just the single pin in question.

Take your multimeter and touch the red lead to PIN A to check for voltage. If you don’t have a power signal here, make sure that your ignition key is turned to the ON position. If you have power at this pin, move on and check PIN B for the low reference ground signal.

Ford Fusion P0135 Trouble Code 5

PIN B should return a ground signal and if you have both power and ground at these two wires, this means that your primary Ford 02 sensor should be working. If your Ford Fusion P0135 is still present, this means that the oxygen sensor has failed and requires a replacement. Once you have replaced your Ford Fusion oxygen sensor, use a scan tool to clear your OBDII trouble code.

Have any questions about our Ford Fusion oxygen sensor heater circuit DIY tutorial? Leave us a comment and let us know below.

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