Rear O2 Sensor Heater Test (1998-2001 2.5L Ford Ranger)

By: Abraham Torres-Arredondo
Created: 19 April 2018
Updated: 21 March 2024
Articl Id: 791

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TEST 2: Making Sure That Ground Is Present

Terminal 2 Should Have Ground Present At All Times. Testing The Rear O2 Sensor's Heater (1998, 1999, 2000, 2001 2.5L Ford Ranger And Mazda B2500)

In this test section, we're going to make sure that the white with black stripe (WHT/BLK) wire of the O2 sensor's engine wiring harness connector is supplying Ground.

This Ground is supplied by the fuel injection computer itself. So be careful not to short this wire to battery voltage or you're going to fry the computer.

The multimeter voltage test I'm suggesting, in the instructions below, is a safe way to make sure Ground is present in this wire.

NOTE: This test is done on the engine wiring harness connector. This connector has round female terminals.

These are the test steps:

1
Disconnect the rear O2 sensor from its harness connector.
2
Locate the WHT/BLK wire of the engine wiring harness oxygen sensor connector.
3
With your multimeter in Volts DC mode, probe the WHT/BLK with the black multimeter test lead.

Ground the red multimeter test lead directly on the battery's positive (+) terminal.
4
Your multimeter should register 10 to 12 Volts DC if Ground is present.

Let's take a look at your test results:

CASE 1: The rear O2 sensor heater element is getting Ground. This is the correct test result.

Now that you know power and Ground are present at the connector, the next step is to check the heater element's internal resistance with a multimeter in Ohms mode. For this test go to: TEST 3: Testing The Resistance Of The Heater Element.

CASE 2: The rear O2 sensor heater element IS NOT getting Ground. Without Ground the heater element will not activate.

It's been my experience that a lack of Ground in the white with black stripe (WHT/BLK wire is usually due to an open circuit in wire itself (between the O2 sensor connector and the fuel injection computer).

You're next step is to check the continuity of this wire between both connectors. The following O2 sensor circuit diagram will help you: Oxygen Sensor Circuit Diagram (1998-2001 2.5L Ford Ranger).

TEST 3: Testing The Resistance Of The Heater Element

Testing The Resistance Of The Heater Element. Testing The Rear O2 Sensor's Heater (1998, 1999, 2000, 2001 2.5L Ford Ranger And Mazda B2500)

Now that you've made sure that the rear O2 sensor's heater is being supplied with power and Ground, the next and last test is to check the heater element's resistance.

This test simply involves checking your Ford Ranger's rear O2 sensor (HO2S 12) heater's resistance with a multimeter in Ohms mode. If the heater element is good, then it should have a resistance between 3 to 30 Ohms.

NOTE: This test is done on the rear O2 sensor's connector itself. This connector has round male terminals (see photo above).

These are the test steps:

1
Locate the O2 sensor terminals 1 and 2 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
2
With your multimeter in Ohms mode, probe terminals 1 and 2 of the O2 sensor itself.
3
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 resistance is within the indicated specification. This is the correct test result and with it you can conclude that your Ford Ranger's rear O2 sensor's heater element is not defective.

CASE 2: The resistance IS NOT within the indicated specification. You can conclude that the rear O2 sensor's heater element is defective. Replace the rear O2 sensor with a new one.