Diagnosing the left front oxygen sensor (HO2S 21) and/or diagnostic trouble code (DTC) P0141: 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 P0141 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.
Contents of this tutorial:
- What Does Trouble Code P0141 Mean?
- How To Resolve Trouble Code P0141.
- Circuit Descriptions Of The Upstream Oxygen Sensor.
- Where To Buy The Oxygen Sensor And Save.
- 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.
- More 4.2L V6 Ford F150 And F250 Diagnostic Tutorials.
You can find this tutorial in Spanish here: Probando El Código P0141 (1997-1998 4.2L V6 Ford F150 And F250) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.2L V6 Ford F150: 1997, 1998.
- 4.2L V6 Ford F250: 1998.
WIRING DIAGRAM: The oxygen sensor circuit wiring diagram can be found here:
P0135 DIAGNOSTIC TESTS: If you need to troubleshoot a P0135 DTC (the right front O2 sensor -HO2S 11), take a look at the following tutorial:
E-SERIES VANS: The P0141 diagnostic tutorial for the 1997-1998 4.2L V6 Ford E150/E250 vans can be found here:
What Does Trouble Code P0141 Mean?
Here's the nitty-gritty of diagnostic trouble code P0141 and the info you need to troubleshoot it:
P0141: Heater Circuit Malfunction (Bank 2, Sensor 1)
- What Does It Mean: This code indicates there's an issue with the heater circuit of the oxygen sensor situated upstream from the catalytic converter on the engine side where cylinder number 4 is located.
- Bank 2 Sensor 1 (B2S1) Location: You'll find this oxygen sensor on the exhaust manifold that connects to the left (driver) side of the engine. This side of the engine is referred to as 'Bank 2' and contains cylinders 4, 5, and 6 (see illustration above).
This O2 sensor is responsible for measuring the oxygen content in the exhaust gases as they exit cylinders 4, 5, and 6, before passing through the catalytic converter. The powertrain control module (PCM) uses this data to fine-tune the air/fuel mixture in real-time for the three cylinders belong to Bank 2. - Other Designations: This particular O2 sensor goes by different names. Here are the most common ones you'll come across:
- Upstream Left Oxygen Sensor.
- Left Side Upstream Oxygen (O2) Sensor.
- Driver Side Front Oxygen Sensor.
- Left Front O2 Sensor.
- Left Front Heated Oxygen (O2) Sensor.
- Bank 2 Pre-Catalytic Converter Oxygen Sensor.
- HO2S 21.
- Oxygen Sensor Bank 2 Sensor 1.
- Possible Causes:
- Faulty oxygen sensor (Bank 2, Sensor 1).
- Damaged wiring or connector to the sensor.
- PCM malfunction (less common).
How To Resolve Trouble Code P0141
For most people, the 'quick' fix for DTC P0141 is replacing the oxygen sensor that it's flagging as having a heater circuit issue. Here are the top three reasons why:
- O2 Sensor Is Easy To Replace: Swapping out the O2 sensor with a new one isn't difficult. You won't need fancy/expensive diagnostic tools or special skills, just an O2 sensor socket/wrench tool.
- O2 Sensor Is Not Expensive: Compared to other engine management system components, O2 sensors are pretty inexpensive, generally going for around $30 to $50. This makes replacing it the go-to solution for many.
- Lack Of Diagnostic Experience: Diagnosing the heater circuit can be time-consuming and you might need a high-end scan tool (the ones that cost $3,000 or more). Plus, you might need access to professional diagnostic information.
For most folks (even some repair shops), it's more practical to replace the sensor right off the bat. The thinking here is that it's not very common to have other issues, such as electrical wiring problems in the O2 sensor heater circuit.
If you want to skip testing the O2 sensor's heating element and just replace it, I have one important suggestion:
- Inspect Wiring Harness: Visually inspect the oxygen sensor's wires, including both the wires coming from the sensor and the engine wiring harness wires that connect to them. Check for any signs of damage, like melted insulation or wires that are shorted together.
Damaged wiring can prevent the engine from starting. This happens because the fuse supplying power to the O2 sensor's heating element also powers the fuel pump, fuel injectors, and ignition coil. A short circuit in the O2 sensor wiring can blow this fuse.
If you find any issues with the wiring, address them before installing a new oxygen sensor.
To see my oxygen sensor recommendations and where to buy them, go here: Where To Buy The O2 Sensor And Save.
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:
| Pin | Wire Color | Description |
|---|---|---|
| 1 | YEL/LT BLU | Heater Ground (-) |
| 2 | RED | Heater Power (+) |
| 3 | GRY/RED | O2 Signal Ground |
| 4 | RED/BLK | O2 Signal |
Where To Buy The Oxygen Sensor And Save
If you find, after testing the upstream oxygen sensor that its heater element is fried, take a look at the links below. I think they'll save you some bucks:
NOTE: If you're not sure if the above upstream O2 sensor fits your particular 4.2L V6 equipped F150 (F250) don't worry, once you get to the site, they'll make sure the sensor fits, if not, they'll find you the correct one.
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 engine wiring harness O2 sensor connector. This connector has female terminals.
OK, this is what you'll need to do:
- 1
Disconnect the O2 sensor from its engine wiring harness connector.
- 2
Place your multimeter in Volts DC.
- 3
Connect the black multimeter test lead to the battery negative (-) post.
- 4
Turn the key to the RUN position but don't crank or start the engine.
- 5
With the red multimeter test lead, probe the male terminal of the connector that connects to the RED wire of the O2 sensor's 4-wire connector.
- 6
Your multimeter should register 10 to 12 Volts DC.
Let's take a look at your test results:
CASE 1: Your multimeter confirms that the RED wire is delivering 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 IS NOT supplying 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.
