Diagnosing the left front oxygen sensor (HO2S 21) and/or diagnostic trouble code (DTC) P0151: HO2S 21 Circuit Malfunction is not as hard as you might think. You don't need any expensive diagnostic test equipment, all you'll need is a multimeter.
A trouble code P0151 tells you that the internal heater for the right front oxygen sensor has a problem. This usually happens when the heater (inside the left front O2 sensor) goes bad.
The cool thing is that the O2 sensor can be tested to be sure the heater element is fried. I'll show you the 3 basic tests you'll need to perform to find out.
NOTE: This oxygen sensor is known by several different names:
- Left Front Heated Oxygen (O2) Sensor.
- HO2S 21.
- Left Side Upstream Oxygen (O2) Sensor.
- Oxygen Sensor Bank 2 Sensor 1.
If you need to troubleshoot the right front O2 sensor (HO2S 11), take a look at the following tutorial:
Contents of this tutorial:
- Circuit Descriptions Of The Upstream Oxygen Sensor.
- TEST 1: Verifying The Heater Element Is Getting Power.
- TEST 2: Verifying The Heater Element Is Getting Ground.
- TEST 3: Testing The Heater Element's Resistance.
- Location Of The Oxygen Sensors.
- Where To Buy The Oxygen Sensor And Save Some $$$.
- More 4.2L Ford Diagnostic Tutorials.
You can find this tutorial in Spanish here: Probando El Código P0151 (1997-1998 4.2L Ford E150, E250, F150) (at: autotecnico-online.com).
Circuit Descriptions Of The Upstream Oxygen Sensor
As you've already found out, the left front heated oxygen sensor (HO2S 21) is a 4 wire oxygen sensor.
The oxygen sensor is two components in one. One part does the actual sensing of the oxygen content of the exhaust. That part uses 2 of the 4 wires. The other 2 wires supply the heater with power and Ground.
O2 SENSOR LOCATION: You can find the location of the left front oxygen sensor (HO2S 21) here: Location Of The Oxygen Sensors.
Below, you'll find the color of the wires of the engine wiring harness oxygen sensor connector for sensor HO2S 21:
|Left Upstream Oxygen Sensor (HO2S 21) Pinout (1997-1998 4.2L Ford F150, E150, E250)|
|1||YEL/LT BLU||Heater Ground (-)|
|2||RED||Heater Power (+)|
|3||GRY/RED||O2 Signal Ground|
TEST 1: Verifying The Heater Element Is Getting Power
The first thing we'll check is that the left front O2 sensor (HO2S 21) is getting power. This power is in the form of 12 Volts DC.
The red (RED) wire, of the HO2S 21 engine wiring harness connector, is the one that feeds power to the HO2S 21 heater element.
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!
IMPORTANT: The illustration of the connector above is of the connector on the oxygen sensor itself. To check for power, you need to test the RED wire of the engine wiring harness sensor connector.
OK, this is what you'll need to do:
Locate the upstream oxygen sensor and disconnect it from its harness connector.
Locate the RED wire of the engine wiring harness oxygen sensor connector.
With your multimeter in Volts DC mode, probe the RED wire with the red multimeter test lead.
Ground the black multimeter test lead directly on the battery negative (-) terminal.
With the Key On Engine Off (KOEO), the RED wire should have 10 to 12 Volts DC.
Let's take a look at your test results:
CASE 1: Your multimeter confirms that the RED wire has 10 to 12 Volts DC. So far so good since this test result confirms that the upstream oxygen sensor's heater element is getting power.
The next step is to make check that the YEL/LT BLU wire, of the O2 sensor engine wiring harness connector, is feeding Ground to the heater element. For this test, go to: TEST 2: Verifying The Heater Element Is Getting Ground.
CASE 2: Your multimeter confirms that the RED wire DOES NOT have 10 to 12 Volts DC. Re-check that you're testing the correct wire and that the key is in the RUN position (but don't crank or start the engine) and re-test.
If you still don't see 10 to 12 Volts DC, then this test result tells you that the upstream oxygen (O2) sensor itself IS NOT bad since without power, the heater element won't work.
Although it's beyond the scope of this article, the next step is to find out why this battery power is missing using a wiring diagram.