Left Front O2 Sensor (HO2S21) Heater Test (1997-1998 F150 - F250)

Left Front O2 Sensor (HO2S11) Heater Test (1997-1999 F150 - F250)

Testing the oxygen sensor for a heater circuit malfunction (specifically trouble code P0141: Heated Oxygen Sensor (HO2S-21) Circuit Malfunction) is pretty easy and can be done with a simple multimeter.

All that's involved is making sure the O2 sensor's heater is getting both power and Ground. The last test is to make sure its resistance is within factory specification. Don't worry, I'll show you how to do all 3 tests it in a step-by-step manner in the following pages.

As you're probably already aware, your Ford has a total of 4 oxygen sensors. Each one is pretty much tested in the same way. What changes between them is the color of the wires of the O2 sensor engine wiring harness connector. For this specific reason, this tutorial focuses on testing the left front oxygen sensor (usually labeled as: HO2S21).

In Spanish You can find this tutorial in Spanish here: Calentador del Sensor de Oxígeno -P0141 (1997-98 F150-F250) (at: autotecnico-online.com).

NOTE: This tutorial covers only the 1997-1998 Ford F150 and F250 equipped with a 4.6L/5.4L V8 engine.

The following tutorial may also be of help:

Right Front Oxygen (HO2S21) Sensor Circuit Descriptions

Left Front Oxygen (HO2S21) Sensor Circuit Descriptions

If you need to see the location of the left front oxygen sensor (HO2S21), take a look at the illustration in section: Oxygen Sensor Locations.

In the following pages of this tutorial, I'm gonna' refer to 2 O2 sensor connectors. The difference may seem pretty obvious, but just to make sure here's a breakdown of each:

  • O2 sensor engine wiring harness connector
    • Is the connector that belongs to the entire wiring harness of your Ford's engine.
    • This connector has round female terminals.
  • O2 sensor connector
    • Is the connector that belongs to the oxygen sensor itself.
    • This connector has round male terminals.

Below is a description of the wires of the left front oxygen sensor (HO2S21):

Left Front O2 Sensor (HO2S21) Circuit Descriptions
Pin Wire Color Description
1 RED VPWR -12 Volts
2 YEL/LT BLU Heater GND -Heater Ground Provided Internally by the PCM
3 GRY/RED O2 SIG RTN -Sensor Signal Ground
4 RED/BLK O2 SIG -Oxygen Sensor Signal

Important Safety Precautions When Testing The O2 Sensor

As I mentioned earlier, testing the left front O2 sensor itself with a multimeter is a pretty simple and straightforward process. The hard part is getting to the sensor.

This is because you'll need to lift your Ford up with a jack to get to the oxygen sensors. Not only that, the sensors are bolted into the exhaust pipes and these get very, very hot (when the engine is running).

This means you have to take the following safety precautions:

  • Place the vehicle on jack stands.
    • Never trust the jack to hold up the vehicle!
  • Let the engine cool down completely before attempting the oxygen sensor tests.
    • The O2 sensor gets very hot! It stays hot a long time after engine shutdown. So to avoid getting burned, let your vehicle's engine cool down completely.

Be careful, use common sense and think safety all of the time while working around your vehicle.

Where To Buy The Oxygen Sensor And Save

If you're looking for the original Motorcraft® oxygen sensor, the best place to buy it is online (it's a whole lot cheaper).

The following links will help you comparison shop for the oxygen sensors:

Not sure if the above oxygen sensor fits your particular Ford? Don't worry, once you get to the site, they'll make sure it fits. If it doesn't, they'll find you the right one.

Ford Vehicles:

  • F150 4.6L
    • 1997, 1998
  • F150 5.4L
    • 1997, 1998
  • F250 4.6L
    • 1997, 1998
  • F250 5.4L
    • 1997, 1998