TEST 2: Manually Inducing A Lean Condition
Just like we tested how the sensors react to a rich air/fuel mixture, we can also check their response to a lean condition.
The fastest way to force a lean mixture is by allowing unmetered air into the intake system —and the simplest place to do that is by slightly loosening the large vacuum hose connected to the brake booster check valve.
This hose pulls in a significant amount of air when loosened. That sudden influx of extra air creates a lean condition that your upstream O2 sensors should immediately detect.
IMPORTANT: Don't fully disconnect the vacuum hose from the brake booster. Doing so will cause the engine to stall.
What you want to do is just loosen it a bit —enough to let some unmetered air in, causing the mixture to go lean, but not enough to kill the engine. You should hear a light hissing sound when it's loose enough.
IMPORTANT: Do a trial run, with the engine off, of loosening the vacuum hose that connects to the brake booster. If it has never been removed, it's gonna be stuck and you want to avoid having to wrestle with it with the engine running.
Here's how to run the test:
- 1
Start the engine and let it idle for around 15 minutes.
- 2
Plug in your scan tool and switch to Live Data mode.
- 3
Locate the O2 sensor data labeled O2S11 and O2S21.
- 4
Watch the voltage readings for both sensors.
- 5
Gently pull back or slightly loosen the large vacuum hose connected to the brake booster check valve.
Do this just enough to let in some extra air. If you hear a hissing noise, you're doing it right. - 6
Monitor the O2S11 and O2S21 voltage readings on the scan tool.
The values should drop quickly below 0.200 Volts. In some cases, they might dip as low as 0.050 Volts. - 7
Once the voltages drop and stabilize, press the vacuum hose back into place.
- 8
After a few seconds, the voltage readings should begin oscillating again between 0.100 and 0.900 Volts.
Alright, here's how to interpret your results:
CASE 1: Both O2S11 and O2S21 voltage readings dropped below 0.200 Volts when the hose was loosened. That confirms both upstream oxygen sensors are correctly detecting a lean mixture. They're working as expected.
If you're still seeing oxygen sensor-related trouble codes, the root cause is likely something else. Check this out next: Oxygen Sensor Codes Keep Coming Back.
CASE 2: O2S11 didn't drop below 0.200 Volts during the test. This tells us Bank 1 Sensor 1 (O2S11) isn't reacting to the lean condition. You'll need to replace that sensor.
CASE 3: O2S21 didn't respond with a voltage drop. That means Bank 2 Sensor 1 (O2S21) has failed to detect the lean mix. Go ahead and replace that oxygen sensor.
Oxygen Sensor Codes Keep Coming Back
Maybe you ran the tests and everything checked out fine. Or maybe you already swapped out one or both oxygen sensors. But here you're again —the PCM is still flagging them as bad.
In other words, that check engine light has returned, and you're still seeing O2 sensor-related codes pop up.
When this happens, it's often because the engine is actually running too rich or too lean —but the oxygen sensor isn't the real culprit.
To put it simply: something else is throwing off the air/fuel balance, and the O2 sensor is just the messenger catching the blame.
Here are some of the usual suspects:
- Weak fuel pump: If it can't maintain proper fuel pressure, the engine may run lean, triggering sensor codes.
- Leaky intake manifold gaskets: Allow air to bypass the MAP sensor, creating a lean mix that the PCM can't fully adjust for.
- Dirty or clogged fuel injectors: Can cause certain cylinders to run lean or rich, leading to misfires and confusing O2 sensor feedback.
- Defective MAP sensor (Manifold Absolute Pressure): Can send inaccurate readings to the PCM, throwing off fuel trims in either direction.
- Bad ECT sensor (Engine Coolant Temp): If it reads too cold, the PCM may stay in open loop longer and over-fuel the engine.
- Faulty IAT sensor (Intake Air Temp): A bad reading can make the PCM miscalculate how much fuel is needed, leading to mixture problems.
- Thermostat stuck open or wrong temp rating: Keeps the engine running cool, which may cause the PCM to stay in rich mode longer than necessary.
- Exhaust leak ahead of the O2 sensor: Draws in outside air, giving the sensor a false lean reading and tricking the PCM into overcompensating.
All of these issues can push the PCM into either adding too much fuel or not enough —and that imbalance is what makes the oxygen sensor appear to be failing, even when it's doing its job.
The tricky part? These kinds of problems can be intermittent. They might not show up when you're actively testing —especially if the component is just beginning to fail.
What can you do? Give it a little time. Let the issue develop. Once it becomes more consistent, you'll have a much better chance of catching it in action with your scan tool. That's when you'll finally be able to pinpoint it, confirm it, and fix it for good.
More 5.2L, 5.9L V8 Dodge Ram Van Test Articles
I've written quite a few 5.2L, 5.9L V8 Dodge Ram van 'how to' tutorials. You can find all here:
Here's a sample of the tutorials you'll find in the index:
- How To Test The MAP Sensor (1998-2003 5.2L, 5.9L V8 Dodge Ram Van).
- How To Test The Intake Air Temp (IAT) Sensor (1998-2003 5.2L, 5.9L V8 Dodge Ram Van).
- How To Test Engine Compression (1989-2003 5.2L, 5.9L V8 Dodge Ram Van).
- How To Test For A Blown Head Gasket (1989-2003 5.2L, 5.9L V8 Dodge Ram Van).
If this info saved the day, buy me a beer!
