P0135 Ford 2.3L – Test a Ranger 02 Sensor

The P0135 Ford OBDII check engine trouble code signals an issue with your upstream oxygen Ranger 02 Sensor. The specific error code for this Ford OBDII trouble code is Oxygen 02 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1). The heater circuit powers the oxygen sensor and enables it to accurately read the air fuel ratio in your exhaust gases. I’ll be showing you how to test your 02 sensor in a 2.3 liter Ford.

Our test vehicle today is a 1996 Ford Ranger with the 2.3 liter 4 cylinder engine in it, however this engine is also found in the Mustang and Mazda B2300. In order to test your 02 sensor, you’ll need to locate the sensor that’s having the problem. In our specific test scenario, it’s Bank 1 Sensor 1, which is upstream or before your catalytic converter. This can also be called the front 02 sensor or pre-cat sensor.


Because our OBDII P0135 Ford check engine trouble code signifies the circuit malfunction, we be testing the heater circuits only in our Ford Ranger. The front oxygen sensor in our 2.3 liter Ford is part number 5L8Z-9G444-A or 5L8Z-9G444-G and can also be 5L8Z-9G444-L. This front 02 sensor also carries a few interchange part numbers, listed below :

  • F0TZ-9F472-C
  • F0UZ-9F472-C
  • F57Z-9G444-C
  • F5AF-9G444-CB
  • F6DZ-9G444-C
  • YL8Z-9G4444-CA
  • YL8Z-9G444-AC

What could be wrong?


Because of the very specific nature of the P0135 OBDII trouble code, there isn’t much to diagnose to your problem. Either your Ranger 02 Sensor has failed or the wiring to your oxygen sensor has an issue.

Where is my Bank 1 oxygen sensor?


The Bank 1 02 sensor is located ahead of your catalytic converter, directly mounted by your exhaust headers. In order to test your 02 sensor, you will need to unplug the front or primary 02 sensor. Using a multimeter you will be probing the WIRING harness side of the oxygen sensor, not the sensor side.


Sensor side of the harness

As a reference check the image above for the harness connector and 2.3 liter Ford oxygen sensor wiring pinout. Remember you will be testing the engine side of the harness using your multimeter to fix your P0135 check engine code.

Check Bank 1 oxygen sensor for Power

In order to begin testing your 2.3 liter Ford oxygen sensor, connect the black test lead of your multimeter to the negative terminal on your battery. Now gently probe the front of the Bank 1 02 sensor wiring connector at PIN B for 12 volts from the ECU. Nothing at this wire? measure PIN A on the same connector for a low reference ground signal. If your 2.3 liter oxygen sensor is getting ground, double check PIN B for power.

If you do not receive 12 volts at this pin, stop now because there’s something wrong with your wiring harness or charging system.

Double check your battery voltage and check your alternator to ensure that both are within proper working parameters. Repeat this step until you know that your Bank 1 oxygen sensor is getting power.

Check Bank 1 oxygen Ranger 02 Sensor heater circuit

The next step to fixing your P0135 check engine code is using the same multimeter and probing SENSOR side of the harness this time. Since you’ll be checking the sensor itself, make sure you aren’t measuring the resistance in your Ford wiring harness, but the sensor side.

Since you’ll be using your multimeter to measure the resistance in the oxygen sensor element, probe both PIN A and PIN B on the oxygen sensor. You should read a value that’s within the manufacturer’s specifications for this oxygen sensor, if it’s out of range you’ll require a replacement.

The internal resistance in this primary oxygen sensor should be between 3 – 29 Ω with the Ford 02 sensor at room temperature. If your multimeter shows an open circuit or a ohm reading that’s outside of this range, you’ll require a replacement oxygen sensor. The part numbers to the primary 02 sensor are listed above, once you have the oxygen sensor installed, use a scan tool to clear your trouble code.

Once you have your front oxygen sensor replaced, you’ll be ready to erase your check engine code and fix your P0135 Ford DTC. That does it for our OBDII Guide to resolving the P0135 check engine trouble code by showing you how to test your 02 sensor in a Ford Ranger.

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