The oxygen sensors located before the catalytic converter (commonly referred to as the front sensors) can be tested to determine whether they've failed or are still functioning properly.
What many folks don't realize is just how simple it is to test these sensors using a basic scan tool that offers live data functionality. Often, a check engine light comes on and a diagnostic trouble code points to an oxygen sensor. At that point, people tend to replace the sensor right away. But more often than not, the problem remains even after the replacement.
If you really want to confirm whether the oxygen sensor is actually faulty, this tutorial will guide you through two easy tests that are done with a scan tool that can display live data.
NOTE: Your V8 Dodge Ram van is equipped with multiple oxygen sensors. In this tutorial, we'll focus specifically on testing the ones located before the catalytic converter —these are known as O2S11 and, if your van has it, O2S21.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- Dodge Ram 1500 Van 5.2L V8: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001.
- Dodge Ram 2500 Van 5.2L V8: 1994, 1995, 1996, 1997, 1998.
- Dodge Ram 1500 Van 5.9L V8: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
- Dodge Ram 2500 Van 5.9L V8: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002.
- Dodge Ram 3500 Van 5.9L V8: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
Important Suggestions And Tips
TIP 1 –Use a Live Data Scan Tool: To follow along with this guide, you'll need a scan tool that can display real-time sensor data.
- You don't need to invest in a factory Dodge scanner or an expensive professional unit (the ones that cost thousands).
- Any basic generic scan tool that lets you monitor live data from the sensors will get the job done.
- Don't have one yet? Looking for one? I recommend checking out the: ZM301 OBD2 Scanner Diagnostic Tool (at: amazon.com).
TIP 2 –This test is done with sensors installed: The method I'll walk you through keeps the oxygen sensors in place —attached to the exhaust— and running normally. No need to remove them for this test.
TIP 3 –Warm up the engine before testing: Be sure the engine reaches full operating temperature first.
Even though the oxygen sensors come with internal heaters, you'll get the most accurate results by running the test when the engine is thoroughly warmed up.
TIP 4 –Rule out other sensor problems first: Don't begin testing if you've got active codes for other engine sensors —like the MAP sensor, engine coolant temperature (ECT) sensor, etc.
Issues with other sensors can affect the air-fuel mixture and give you false readings from the oxygen sensors. Resolve those faults first if you've got them.
Symptoms Of A Bad Oxygen Sensor
The symptoms of a failing oxygen sensor can be easy to miss, as they don't always cause major driveability issues. Still, here are the most frequent signs to watch for:
- Check Engine Light (CEL) is on: You'll likely notice the check engine light lit up on your dashboard.
- Stored O2 sensor trouble codes (DTCs): These are the typical diagnostic trouble codes you'll find saved in the PCM when an oxygen sensor is acting up:
- P0131: Upstream Heated O2 Sensor (O2S11) Circuit Low Voltage (Bank 1).
- P0133: Upstream Heated O2 Sensor (O2S11) Circuit Response Too Slow (Bank 1).
- P0151: Upstream Heated O2 Sensor (O2S21) Circuit Low Voltage (Bank 2).
- P0153: Upstream Heated O2 Sensor (O2S21) Circuit Response Too Slow (Bank 2).
- Bad gas mileage: You might notice you're refueling more often. Without accurate O2 sensor feedback, the PCM struggles to optimize the air-fuel ratio.
- Emissions test failure: If your area requires emissions or smog testing, a faulty upstream sensor will almost certainly cause the vehicle to fail the test.
- Sluggish engine behavior (in some cases): While not always present, you could experience mild hesitation, a rough idle, or a slower-than-normal throttle response —especially if the sensor gets stuck reading too rich or too lean, throwing off the fuel trim.
Oxygen Sensor Basics
Before we jump into the two tests I'm going to walk you through, let's quickly cover the core idea behind how oxygen sensors work on your V8 Dodge Ram van. Understanding this will help make sense of what the tests are checking for.
Here are the key points:
- What the O2 Sensor Does:
- The oxygen sensor plays a vital role in helping the PCM (Powertrain Control Module) fine-tune fuel delivery to the engine.
- It works by producing a voltage signal that reflects how much oxygen is present in the exhaust stream.
- This voltage typically ranges between 0.100 and 0.900 Volts.
- The PCM reads this signal to determine whether the air/fuel ratio is running rich or lean.
- When the Engine Runs Rich:
- If too much fuel is being injected, the exhaust will have less oxygen than normal.
- The sensor detects this and generates a higher voltage —usually over 0.500 Volts, and often hitting 0.900 to even 1.0 volt.
- This tells the PCM: "We're running rich".
- When the Engine Runs Lean:
- Once the PCM detects the rich condition, it cuts back on fuel delivery.
- That results in more oxygen in the exhaust, and the sensor reacts by dropping its voltage —sometimes as low as 0.050 to 0.100 Volts.
- Anything under 0.500 Volts signals a lean mixture.
- The PCM then increases fuel flow to bring things back into balance.
- How the cycle works:
- This rich-lean detection and fuel adjustment loop:
- Starts with the O2 sensor reporting conditions, and
- Is followed by the PCM adjusting the fuel accordingly.
- This process repeats constantly while the engine is running, as long as everything is functioning properly.
- This rich-lean detection and fuel adjustment loop:
- How to see it in action:
- Using a scan tool with Live Data mode, you can watch the O2 sensor voltage go up and down in real time.
- This is exactly what we'll be looking at during the test procedures.
- What healthy sensor behavior looks like:
- A good oxygen sensor will rapidly switch between rich and lean signals.
- You'll see the voltage cycling up and down several times each second.
- If the reading stays stuck at one value —or barely moves — the sensor is probably no longer working correctly.
When the upstream O2 sensors (known as O2S11 and O2S21) go bad:
- They stop providing feedback on whether the exhaust is rich or lean,
- Which means the PCM can't adjust the air/fuel mixture accurately.
- This can result in poor engine performance, increased fuel consumption, and failing an emissions test.
Where Are O2S11 And O2S21 Located?
NOTE: Depending on who you're talking to —or what brand of scan tool you're using— the front oxygen sensors can go by several different names:
- You might hear them called "upstream", "pre-cat", "front", or simply "sensor 1".
All these labels refer to the same basic location: the sensor positioned ahead of the catalytic converter.
The number of upstream oxygen sensors on Dodge 5.2L and 5.9L engines isn't always the same —some vans are equipped with one, others with two. Because this varies by vehicle configuration, the most reliable way to know is to examine your exhaust system directly.
You'll find upstream O2 sensors installed in the exhaust pipe near the engine, specifically at the point where the pipe bolts to the exhaust manifold. In short, they sit ahead of the catalytic converter, between the manifold flange and the cat.
When you're viewing Live Data on your scan tool, the upstream or pre-cat sensors are generally labeled as:
- O2S11 –Bank 1 Sensor 1.
- O2S21 –Bank 2 Sensor 1.
O2S11 – Bank 1 Sensor 1:
- For 5.2L and 5.9L V8 engines:
- Bank 1 includes cylinders 1, 3, 5, and 7.
- This bank of cylinders is on the driver side of the van.
- This is where you'll find the O2S11 sensor mounted.
- NOTE: If your Dodge van was built between 1995 and 1999, O2S11 will be the only pre-cat sensor it has.
O2S21 –Bank 2 Sensor 1:
- For 5.2L and 5.9L V8 engines:
- Bank 2 includes cylinders 2, 4, 6, and 8.
- This bank of cylinders is on the passenger side of the van.
- That's where the O2S21 sensor is located.
- NOTE: If your Dodge was made in 2000 or later, it will be equipped with both O2S11 and O2S21 sensors.
Wondering about the rear O2 sensors? Just so you know: the downstream oxygen sensors —those located after the catalytic converter— are labeled as follows:
- O2S12 –Bank 1 Sensor 2.
- O2S22 –Bank 2 Sensor 2.
These rear sensors aren't involved in managing fuel delivery. Their main purpose is to monitor the efficiency of the catalytic converters —so we won't be covering them in this guide.
TEST 1: Manually Inducing A Rich Condition
To begin testing the oxygen sensors, the first step is to create a rich fuel condition on purpose.
The quickest and most effective way to do this is by carefully spraying a small amount of carburetor cleaner into the engine (while it's running) through a vacuum hose that pulls in engine vacuum.
Once the engine burns the carb cleaner inside the cylinders, it temporarily enriches the air/fuel mixture. This should immediately cause the O2 sensor readings to jump to around 0.900 Volts on your scan tool.
Don't have a scan tool yet? Looking for a reliable one? Take a look at my recommended pick: ZM301 OBD2 Scanner Diagnostic Tool (at: amazon.com).
Here's a step-by-step breakdown of what you need to do:
- 1
Start the engine and let it idle for about 15 minutes.
- 2
Hook up your scan tool and switch to Live Data mode.
- 3
Scroll through the list of PIDs until you find O2S11 and O2S21.
These represent the voltage signals coming from the upstream oxygen sensors.
NOTE: If your Dodge is a 1996–1999 model, you'll only see O2S11 listed. The second sensor, O2S21, is only present in models from 2000 onward. - 4
Watch the voltage values for O2S11 and O2S21 on the scan tool.
With the engine warm and idling, these voltages should fluctuate continuously, usually between 0.200 and 0.900 Volts, switching multiple times per second.
If the numbers look stuck or barely move, don't worry yet —just proceed to the next step. - 5
While the engine is still running, spray a short burst of carburetor cleaner into a vacuum hose with strong suction.
If the engine stalls, no big deal —restart it and try using a smaller amount next time. - 6
Check for a voltage spike on O2S11 and O2S21.
You should see the readings climb quickly to around 0.800–0.900 Volts. As long as you're spraying, the voltages should remain high. - 7
Stop spraying and keep an eye on the sensor voltages.
- 8
The readings should begin to drop back down.
After a few seconds, you should see them start oscillating again between 0.100 and 0.900 Volts, just like before.
That's it for this part of the test —now let's interpret what your results mean:
CASE 1: If both O2S11 and O2S21 jumped up to 0.900 Volts during the spray test. This confirms that both upstream sensors are working and responding correctly to a rich condition.
Next, you'll want to check if the sensors can also detect a lean condition. Continue to: TEST 2: Manually Inducing A Lean Condition.
CASE 2: If O2S11 didn't spike to 0.900 Volts. That means the Bank 1 Sensor 1 (O2S11) is not reacting properly. It's time to replace that sensor.
CASE 3: If O2S21 didn't spike to 0.900 Volts. This indicates that Bank 2 Sensor 1 (O2S21) has failed. Replacing that sensor will be necessary.