Rear Oxygen Sensor Heater Test -P0141 (2001-2004 2.0L Ford Escape)

If a P0141: HO2S 12 Circuit Malfunction code is lighting up the check engine light on your 2.0L Ford Escape or Mazda Tribute, more than likely the downstream O2 sensor's heater is toast.

In this tutorial, I'm going to show you how to check that the rear O2 sensor's heater element is getting VPWR power, its HEATER Ground activation signal from the PCM, and that its internal resistance is within spec.

All three of these tests are done with just a multimeter. You don't need a scan tool or any other expensive diagnostic equipment.

With your test results, you'll quickly and easily find out if the heater element in the rear O2 sensor of your Escape or Tribute is fried and needs replacement.

NOTE: The rear oxygen sensor is known by several different names:

1. Post-Catalytic Converter Oxygen (O2) Sensor.
2. HO2S 12 (Heated Oxygen Sensor Bank 1 Sensor 2).
3. Downstream Oxygen (O2) Sensor.
4. Oxygen Sensor Bank 1 Sensor 2.

APPLIES TO: This tutorial applies to the following vehicles:

• 2.0L Ford Escape: 2001, 2002, 2003, 2004.
• 2.0L Mazda Tribute: 2001, 2002, 2003, 2004.

P0135 CODE DIAGNOSTIC TESTS:

• Oxygen Sensor Heater Test -P0135 (2001-2004 2.0L Ford Escape).

Should I Just Replace the Sensor Without Testing It?

When a P0141 lights up the check engine light, most folks skip testing the sensor and just replace it. And I'll be honest with you, that often solves the problem —the heater element is usually the culprit, and a new sensor clears the code.

This "replace it first" approach makes sense for a lot of people because:

• No multimeter on hand —they can't check the heater's power feed, activation/Ground, or resistance.
• Its cheaper than taking it to the shop —even if it doesn't fix the issue, they avoided a diagnostic fee and shop labor and treat the part cost as their DIY diagnostic step.

If you want to go ahead and replace the sensor, I'll point you to two rear O2 sensors from brands I've trusted for years. But before you order or bolt one in, take a quick look at the sensor's wiring and connector and check for:

• Wiring damage:
  • Check if the sensor's wires have melted against the exhaust and are shorted together.
  • See if the insulation's rubbed through on a sharp edge, the copper strands of the wire are exposed.
• Connector issues: Look for corrosion, loose terminals, or broken connector that can keep the heater from getting power and its activation signal.

WHERE TO BUY: The following two rear O2 sensors are from automotive brands I have been using for years and don't hesitate to recommend:

• Walker Products 250-24070 4-Wire Oxygen Sensor (Amazon affiliate link).
• Delphi ES20039 Oxygen Sensor (Amazon affiliate link).

That said, I want to make the case for testing the rear O2 sensor. Here's why:

• You only need a multimeter —No scan tool or fancy gear required.
• The checks are easy to do —All you need to do is verify power and the PCM-controlled Ground/activation signal, then measure heater resistance and compare to spec.
• Easy access —The rear O2 sensor is pretty accessible.
• Step-by-step guidance —In the next sections, I'll walk you through each test so you'll know if the sensor's heater is toast.

Circuit Descriptions Of The Downstream Oxygen Sensor

The rear O2 sensor on your 2.0L Ford Escape or Mazda Tribute is a four wire sensor. Two of the wires belong to the heater element circuit itself, and the other two belong to the oxygen sensing portion of the sensor.

In the table below, you'll find a brief description of each circuit, which will help us to test the heater in the tests to follow.

Downstream Oxygen Sensor Pinout (2001-2004 2.0L Ford Escape)
Pin	Wire Color	Description
1	Light blue with orange stripe (LT BLU/ORG)	VPWR —Heater 12 Volts
2	White with black stripe (WHT/BLK)	H2OS HEATER —Heater activation signal
3	Orange (ORG)	SIG RTN —O2 Signal Ground
4	Red with light green stripe (RED/LT GRN)	H2OS SIG —O2 Signal

NOTE: The O2 sensor connector (the one that belongs to the engine wiring harness) on your Ford Escape or Mazda Tribute has one large index tab and two small index tabs. The location of the small tabs may differ from the illustration shown in this tutorial. Always use the large index tab as your reference point when making your multimeter connections and tests.

Where To Buy The Oxygen Sensor And Save

If your 2.0L Ford Escape (2.0L Mazda Tribute) needs a new downstream oxygen sensor, take a look at the links below. I think they'll save you some bucks:

NOTE: If you're not sure if the above downstream O2 sensor fit your particular 2.0L Escape (2.0L Mazda Tribute) don't worry. Once you get to the site, they'll make sure the sensor is the right one, if not, they'll find you the right one.

TEST 1: Verifying The Heater Element Is Getting Power

All right, the very first thing that we're going to do is make sure that the heater element of the rear O2 sensor is getting power. This power should be available at female terminal 1 of the engine wire and harness connector.

These 12 Volts are provided by fuse 5 of the battery junction box and are delivered to terminal 1 by the light blue with orange stripe (LT BLU/ORG) wire of the sensor's engine wiring harness connector.

CAUTION: Make sure the engine is completely cold engine to avoid getting burned from a hot O2 sensor or exhaust manifold! Be careful and take all necessary safety precautions!

IMPORTANT: This test is done on the engine wiring harness connector which has female terminals.

TERMINAL ID: The O2 sensor connector (the one that belongs to the engine wiring harness) on your Ford Escape or Mazda Tribute has one large index tab and two small index tabs. The location of the small tabs may differ from the illustration shown in this tutorial. Always use the large index tab as your reference point when making your multimeter connections and tests.

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

1. 1

   Disconnect the downstream O2 sensor from its engine wiring harness connector.

2. 2

   Turn the key ON but don't start the engine.

3. 3

   Set your multimeter to Volts DC mode.

4. 4

   Ground the black multimeter test lead directly on the battery negative (-) terminal.
   
   You may need to use a long jumper wire to extend the reach of your multimeter test lead.

5. 5

   Probe female terminal 1 with the red multimeter test lead.
   
   NOTE: Make sure that terminal 1 connects to the LT BLU/ORG wire of the connector.

6. 6

   Your multimeter should register 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: Your multimeter registered 10 to 12 Volts at terminal 1. Perfect. That's the VPWR 12V feed you're looking for, and it means the heater circuit inside the downstream O2 sensor is actually being powered.

Next, we need to see if the WHT/BLK wire is completing the circuit by supplying Ground to the heater element. Go to: TEST 2: Verifying The Heater Element Is Getting Ground.

CASE 2: The multimeter doesn't register 10 to 12 Volts DC at terminal 1. Double-check you're testing the correct terminal, confirm the key is ON in the RUN position, and repeat the test.

If you still don't see the 12V VPWR feed, you've ruled out the O2 sensor itself as the cause of the P0141 code. Without this VPWR voltage, its heater won't switch on.

Pull fuse 5 in the Battery Junction Box and check if it's blown. Once that fuse and circuit are back feeding power to the LT BLU/ORG wire, the rear O2 sensor heater is going to come alive again.

TEST 2: Verifying The Heater Element Is Getting Ground

Now that you've confirmed that the rear O2 sensor is getting VPWR power from terminal 1, we're now going to make sure that terminal 2 is providing the HEATER Ground that activates the heater itself.

This HEATER Ground is provided by the PCM via the white with black stripe (WHT/BLK) wire of the engine wire and harness connector.

Since this Ground is actually the heater's activation signal and it's provided by the PCM, it's only available with the key in the RUN or START position. For our tests, we're going to check it with the key in the RUN position but with the engine OFF.

CAUTION: The HEATER Ground is provided internally by your Escape or Tribute's PCM, so be careful not to connect this circuit to battery 12 Volts or you'll deep-fry the PCM. The multimeter voltage test I'm outlining below is a safe way to check for this PCM-supplied Ground.

NOTE: The H2OS HEATER circuit test takes place on the engine wiring harness connector (which has female terminals) and NOT on the connector that belongs to the O2 sensor itself.

These are the test steps:

1. 1

   Turn the key ON but don't start the engine.

2. 2

   Set your multimeter to Volts DC mode.

3. 3

   Connect the red multimeter test lead directly on the battery positive (+) terminal.
   
   You may need to use a long jumper wire to extend the reach of your multimeter test lead.

4. 4

   Gently probe the female terminal 2 with the black multimeter test lead.
   
   NOTE: Confirm that terminal 2 connects to the WHT/BLK wire of the connector.

5. 5

   The multimeter should register 10 to 12 Volts DC.

Let's see what your test result is telling us:

CASE 1: HEATER Ground is present. Good. That tells you the PCM is doing its job and providing the path the heater circuit needs to work.

Now that you've confirmed both VPWR power and HEATER Ground are present, the next thing is checking the heater element itself. Go to: TEST 3: Testing The Heater Element's Resistance.

CASE 2: HEATER Ground IS NOT present. Make sure you're on the correct terminal, the key is in RUN, and repeat the test.

If the reading still shows no Ground, that rules out the rear O2 sensor as the cause of the P0141 code. Without this PCM-supplied Ground, its heater won't turn on.

More than likely, you're dealing with an open-circuit somewhere in the WHT/BLK wire between the O2 sensor connector and the PCM itself.

TEST 3: Testing The Heater Element's Resistance

So far, your previous tests have confirmed that:

• A P0141 trouble code is lighting up the check engine light on your 2.0L Ford Escape's instrument panel.
• The sensor's heater is getting 12 Volts from the LT BLU/ORG wire (TEST 1).
• The sensor's heater is getting Ground from the WHT/BLK wire (TEST 2).

For our final P0141 DTC diagnostic test, we're going to check the heater's internal resistance with our multimeter in Ohms (Ω) mode.

If the heater element is OK (within the O2 sensor), its resistance should be within 3 to 30 Ohms (Ω).

If its internal resistance isn't within specification, then the heater is fried and the rear O2 sensor needs to be replaced to solve the P0141 HO2S 12 Circuit Malfunction code.

NOTE: Just a reminder that the downstream oxygen sensor has to be completely cold before proceeding with this test since the manual calls for the O2 sensor to be at room temperature for the resistance test.

OK, this is what you need to do:

1. 1

   Locate terminals 3 and 4 of the connector that connects directly to the downstream O2 sensor.
   
   NOTE: This test is done on the connector coming out of the O2 sensor itself. This connector has round male terminals.

2. 2

   Set your multimeter to Ohms mode.

3. 3

   With the multimeter test leads, probe male terminals 3 and 4 of the O2 sensor connector.

4. 4

   If all is OK, you should see about 3 to 30 Ωs on your multimeter.
   
   If the heater element is fried, your multimeter will show an open (usually indicated by the letters OL) or a number over 10 K Ωs.

Let's take a look at your test results:

CASE 1: The multimeter shows the heater's resistance is within specification. That's the result you want. It confirms the heater element inside the downstream O2 sensor is intact and working like it should.

This test result also lets you know the sensor itself isn't the cause of the P0141 code.

CASE 2: The multimeter shows the heater's resistance IS NOT within specification. That tells you the downstream O2 sensor's heater element has failed.

Replace the sensor only after making sure these three conditions are true:

• The heater circuit is getting VPWR power (TEST 1).
• The PCM is providing HEATER Ground (TEST 2).
• This resistance test proves the heater element itself is out of spec.

Whenever you're ready to replace the rear O2 sensor, the following two are from automotive brands I have been using for years and don't hesitate to recommend:

• Walker Products 250-24070 4-Wire Oxygen Sensor (Amazon affiliate link).
• Delphi ES20039 Oxygen Sensor (Amazon affiliate link).

More 2.0L Ford Escape Diagnostic Tutorials

You can find a complete list of 2.0L Ford Escape tutorials in this index:

• 2.0L Ford Escape Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

• How To Test Engine Compression (2001-2004 2.0L Ford Escape).
• How To Test The Throttle Position Sensor (2001-2004 2.0L Ford Escape).
• How To Test The Fuel Injectors (2001-2004 2.0L Ford Escape).
• How To Test The Starter Motor (2001-2004 2.0L Ford Escape).