TEST 2: Testing The Heater Element's Resistance
The last test we need to do is to check that the resistance of the heater element is within specification.
The resistance specification of the heater element is: 5 to 7 Ωs.
We'll check the O2 sensor's heater element's resistance with your multimeter in Ohms mode. If the O2 sensor's heater element is not within specification, then you can conclude that the O2 sensor is defective and behind the trouble code 21.
Locate the round male terminals #3 and #4 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
With your multimeter in Ohms mode, probe terminals 3 and 4 of the O2 sensor itself.
Remember, you're testing the round male terminals of the O2 sensor's connector and not the engine wiring harness connector (which has female terminals).
If all is OK, you should see about 5 to 7 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 O2 sensor's heater resistance is within spec. This is the correct and expected test result. Since the O2 sensor's heater element is within specification, you can now conclude that the O2 sensor itself is not the cause of trouble code 21 lighting up your check engine light.
CASE 2: Your multimeter showed an open circuit (OL). This confirms that the O2 sensor's heater element is fried.
Since, up to this point you have confirmed that the O2 sensor is getting power and ground and the heater elements resistance IS NOT within specification, you can conclude the O2 sensor has to be replace with a new one.
Where To Buy The Oxygen Sensor And Save
The following links will help you to comparison shop and hopefully save you a few bucks on the Oxygen sensor for your 2.5L Dodge Dakota.
NOTE: If you're not sure if the above O2 sensor fits your particular 2.5L Dodge Dakota, don't worry, once you get to the site, they'll make sure the sensor switch is the left one, if not, they'll find you the left one.
More 2.5L Dodge Dakota Tutorials
You can see a complete list of 'how to' tutorials for the 2.5L OHV Dodge Dakota in this index: Chrysler 2.5L 4-Cylinder Index of Articles.
Here's a sample of the tutorials you'll find there:
- How To Test The MAP Sensor (1994-1995 2.5L SOHC Dodge Dakota)
- How To Test The TPS (1993-1995 2.5L SOHC Dodge Dakota)
- How To Test Engine Compression (1993-2002 2.5L Dodge Dakota)
If this info really saved the day, buy me a beer!