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

TEST 3: Checking The O2 Heater's Resistance

If you've reached this point, you have: 1) Confirmed that the left front O2 sensor's heater is getting power (TEST 1). 2) That it's getting Ground (TEST 2).

Now, we're gonna' check the internal resistance of the left front O2 sensor's heater element.

NOTE: The illustration above shows the connector of the oxygen sensor itself. The resistance test is done on the O2 sensor's connector and NOT on the O2 sensor engine wiring harness connector.

Alright, these are the test steps:

1. 1

   Locate the two male terminals of the O2 sensor's connector shown in the illustration above.
   
   NOTE: To correctly identify the two terminals, keep in mind the top locking tab on the connector. The orange arrow in the illustration above also refers to this top locking tab.

1. 2

   Place your multimeter in Ohms mode and measure the resistance between the two pins.
   
   Remember, the O2 sensor connector itself has male terminals. The O2 sensor engine wiring harness connector has female terminals.

2. 3

   If all is OK, your multimeter should register 3 to 30 Ohms.
   
   If the heater element is fried, your multimeter will show an open, usually indicated by the letters OL or a resistance in the thousands (K Ohms).

Let's take a look at what your test results mean:

CASE 1: The O2 sensor's heater element is within specification (3 to 30 Ohms). This means that the left front oxygen sensor's heater element IS NOT fried.

CASE 2: The O2 sensor's heater element IS NOT within specification. Double check your multimeter connections and repeat the test.

If after repeating the test your multimeter still DOES NOT indicate the Ford factory manual specification of 3 to 30 Ohms, then the left front O2 sensor's heater is fried. Replacing the left front oxygen sensor will solve the P0141 trouble code.

Oxygen Sensor Locations

NOTE: The above illustration is for dual exhaust Ford V8 and V6 equipped vehicles. The Ford vehicles covered by this tutorial are dual exhaust equipped with a total of 4 oxygen sensors.

The illustration below is for single exhaust Ford V8 and V6 equipped vehicles, I included the illustration below for comparison purposes and to aid you in further locating the left front oxygen sensor (HO2S-11).

More Ford 4.6, 5.4L Tutorials

You can find a complete list of tutorials for 4.6L, 5.4L equipped Fords here: Ford 4.6L, 5.4L Index Of Articles.

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

1. How To Test The Camshaft Position Sensor (1997-1999 Ford 4.6L, 5.4L).
2. How To Test The Blower Motor Resistor (Ford 4.6L, 5.4L).
3. How To Test The Fuel Pump -No Start Test (Ford 4.6L, 5.4L).
4. Coil-On-Plug (COP) Ignition Coil Misfire Tests (Ford 4.6L, 5.4L).
5. How To Diagnose Misfire Codes P0300-P0308 (Ford 4.6L, 5.4L).
6. How To Test The Ford EGR Valve EGR Vacuum Solenoid, DPFE Sensor (at: easyautodiagnostics.com).
7. How To Test The Ford MAF Sensor (4.2L, 4.6L, 5.4L) (at: easyautodiagnostics.com).