How To Test The HO2S-21 Oxygen Sensor's Heater -P0155 (2001, 2002, 2003, 2004 3.0L Ford Escape And 3.0L Mazda Tribute)

This tutorial will help you test the heater element in the HO2S-21 oxygen sensor on the 2001-2004 3.0 Ford Escape (3.0L Mazda Tribute).

Testing the heater element within the O2 sensor is not hard. Best of all, you don't need any expensive diagnostic equipment to do it. I'll show you how to do it with just a multimeter.

Also, you'll be able to easily diagnose a P0155 diagnostic trouble code with the help of this tutorial.

APPLIES TO: This P0155 diagnostic test tutorial applies to the following vehicles:

  1. 3.0L Ford Escape: 2001, 2002, 2003, 2004.
  2. 3.0L Mazda Tribute: 2001, 2002, 2003, 2004.

What Does Trouble Code P0155 Mean?

A P0155: Heated Oxygen Sensor Heater Circuit Malfunction (Bank 2 Sensor 1) trouble code means that there's a problem with the internal heater of the front oxygen sensor for bank 2.

Quite a few things can cause a P0155 trouble code to light up the check engine light on your 3.0L Ford Escape (3.0L Mazda Tribute), it usually boils down to one of the following:

  1. The HO2S-21 oxygen sensor's heater has failed.
  2. The O2 sensor's heater is not getting 12 Volts because its power wire has an open-circuit problem.
  3. The O2 sensor's heater is not getting Ground because its Ground wire has an open-circuit problem.
  4. The wires that supply 12 Volts and Ground to the O2 sensor's heater have shorted together.

The HO2S-21 oxygen sensor is located on the exhaust manifold that faces your Ford Escape's radiator. You can see it's location on the exhaust manifold here: Location Of Oxygen Sensor HO2S-21.

Where To Buy The Oxygen Sensor And Save

The HO2S-21 oxygen sensor is known by a lot of names and this can cause a lot of confusion when it's time to purchase a new one. To help you out, here's the original equipment Motorcraft part number: Motorcraft DY878.

The following links will help you to comparison shop and save a few bucks on the HO2S-21 oxygen sensor:

NOTE: If you're not sure if the above upstream O2 sensor fits your particular Ford Escape (Mazda Tribute), don't worry, I researched the part numbers for you and the above O2 sensor's are the replacements for the HO2S-21 oxygen sensor on the 3.0L Ford Escape and 3.0L Mazda Tribute.

Circuit Descriptions Of The HO2S-21 Oxygen Sensor

As you're probably already aware, the HO2S-21 oxygen sensor has 4 wires sticking out of its connector.

Two of those four wires feed the heater element (within the O2 sensor) with 12 Volts and Ground. It's with these 12 Volts and Ground that the heater element activates within the O2 sensor.

The heater element needs 12 Volts and Ground to activate and start heating the O2 sensor it's a part of

The table below has a brief description of the four wires that connect to the HO2S-21 oxygen sensor.

Here's a brief description of the four wires of the oxygen sensor HO2S-21 connector:

O2 Sensor Bank 2 Sensor 1 (2001-2004 3.0L Ford Escape And Mazda Tribute)
Pin Wire Color Description
1 LT BLU/ORG Heater 12 Volts
2 YEL/LT BLU Heater Ground
3 ORG O2 Signal Ground
4 RED/BLK O2 Signal

NOTE: The HO2S-21 oxygen sensor's engine wiring harness connector has female terminals (see photo 1 of 2 above). The connector on the oxygen sensor itself has male spade terminals (see photo 2 of 2).

In case you're wondering why the oxygen sensor needs an internal heater, it's because to activate, it needs to reach and maintain a constant temperature of 600° Fahrenheit.

Even though the exhaust gas gets very hot, it isn't able to maintain the O2 sensor at a constant 600° Fahrenheit.

This is the reason why the O2 sensor needs and internal heater to help it reach and maintain a constant 600° Fahrenheit temperature.

TEST 1: Making Sure The O2 Sensor's Heater Is Getting 12 Volts

How To Test The HO2S-21 Oxygen Sensor's Heater -P0155 (2001, 2002, 2003, 2004 3.0L Ford Escape And 3.0L Mazda Tribute)

The very first thing that we're going to do, to start our O2 sensor heater element diagnostic, is to make sure that the light blue with orange stripe (LT BLU/ORG) is feeding the heater element with 10 to 12 Volts DC.

The light blue with orange stripe (LT BLU/ORG) wire connects to the terminal labeled with the number 1 in the image above.

The LT BLU/ORG wire gets these 12 Volts from fuse #5 of the Battery Junction Box. (underhood fuse/relay box).

If 10 to 12 Volts DC are present in the LT BLU/ORG wire, then our next step is to make sure that the YEL/LT BLU wire is providing Ground to the heater element.

If 10 to 12 Volts are not present in the LT BLU/ORG wire, then the next step is to make sure that fuse #5 (of the Battery Junction Box) isn't blown.

CAUTION: The oxygen sensor gets and stays very hot even after the engine is off! Perform this test with a completely cold engine. Be careful and take all necessary safety precautions! If you raise your vehicle with a jack, place it on jack stands!

IMPORTANT: To check the presence of 12 Volts, you'll test the oxygen sensor's engine wiring harness connector. This connector has female terminals as shown in the image above.

OK, this is what you'll need to do:

  1. 1

    Locate the HO2S-21 oxygen sensor and disconnect it from its engine wiring harness connector.

  2. 2

    Set your multimeter to Volts DC mode.

  3. 3

    Turn the key ON but don't crank or start the engine (this will power up the O2 sensor's engine wiring harness connector).

  4. 4

    Probe the light blue with orange stripe (LT BLU/ORG) wire of O2 sensor's connector with your multimeter's red test lead

    The LT BLU/ORG wire connects to the female terminal labeled with the number 1 (see photo above).

  5. 5

    With the Key On Engine Off (KOEO), your multimeter should register 10 to 12 Volts DC.

Let's examine your test results:

CASE 1: 10 to 12 Volts are present in the LT BLU/ORG wire. This is the correct and expected test result.

This test result lets you know that the HO2S-21 oxygen sensor's heater element is getting 12 Volts from fuse #5 of the Battery Junction Box (underhood fuse/relay box).

Now we're gonna' make sure that the O2 sensor's heater element is getting Ground. For this test go to: TEST 2: Making Sure The O2 Sensor's Heater Is Getting Ground.

CASE 2: 10 to 12 Volts DC are not present in the LT BLU/ORG wire. This test result usually means that fuse #5, of the Battery Junction Box, is blown.

Your next step is to check the fuse and make sure that it's not blown. If it is blown, replace it and repeat the test.

If fuse #5 is blown, the most likely cause of this problem is that:

  1. The O2 sensor's wires are touching the exhaust pipe and the insulation of the LT BLU/ORG wire has melted, causing it to short to Ground on the exhaust pipe.
  2. The O2 sensor's wires are rubbing against a hard metal edge and the insulation of the LT BLU/ORG wire has been rubbed off, causing it to short to Ground.

Repair any visible heat damage to the wires and after replacing the fuse, repeat the test.

Ford Vehicles:

  • Escape 3.0L
    • 2001, 2002, 2003, 2004

Mazda Vehicles:

  • Sable 3.0L
    • 2001, 2002, 2003, 2004