TEST 2: Manually Inducing A Lean Condition
Just like you tested the sensors' response to a rich condition, we can also test how they react to a lean condition.
The quickest way to create a lean condition is by introducing unmetered air into the intake —and the easiest spot to do this is by loosening the large vacuum hose connected to the Brake Booster check valve.
This hose draws in a lot of air when loosened. That sudden rush of extra air will cause a lean condition that the O2 sensors should detect right away.
IMPORTANT: Do not fully disconnect the vacuum hose from the Brake Booster. If you remove it completely, the engine will most likely stall.
You just want to loosen it slightly —enough to let some unmetered air in and lean out the air/fuel mixture, but not so much that the engine shuts off. You should hear a small hissing sound when it's loose enough.
IMPORTANT: Before starting the engine, do a quick dry run by loosening the vacuum hose connected to the brake booster. If it's never been taken off before, it might be stuck —and you don't want to be fighting with it while the engine's running.
These are the test steps:
- 1
Start the engine running and let run for about 15 minutes.
- 2
Connect your scan tool and enter its Live Data mode.
- 3
Scroll down to the PIDs labeled O2S11 and O2S21.
- 4
Observe the voltage readings for O2S11 and O2S21.
- 5
Gently pull back or loosen the large vacuum hose from the brake booster check valve.
Do this just enough to let unmetered air enter the intake. You should hear a hissing sound. - 6
Watch the O2S11 and O2S21 readings on your scan tool.
The voltage readings should drop quickly below 0.200 Volts. Some sensors may dip as low as 0.050 Volts. - 7
Once the reading drops and stabilizes low, reseat the vacuum hose back onto the check valve.
- 8
After a few seconds, the sensor readings should go back to switching between 0.100 and 0.900 Volts.
OK, the lean test is done, let's take a look at what your test results mean:
CASE 1: The O2S11 and O2S21 voltage readings dropped below 0.200 Volts when you loosened the brake booster vacuum hose. This tells you that both upstream oxygen sensors are reacting properly to a lean condition. They're working fine at this point in time.
If you're still getting oxygen sensor codes, the issue lies elsewhere. Oxygen Sensor Codes Keep Coming Back.
CASE 2: O2S11 did not drop below 0.200 Volts during the lean condition test. This confirms that Bank 1 Sensor 1 (O2S11) is not responding correctly. You can go ahead and replace this oxygen sensor.
CASE 3: O2S21 did not drop below 0.200 Volts during the lean condition test. This confirms that Bank 2 Sensor 1 (O2S21) is not responding correctly. You can go ahead and replace this oxygen sensor.
Oxygen Sensor Codes Keep Coming Back
So you tested the oxygen sensors and they checked out fine —or maybe you already replaced one (or both)— and yet the PCM keeps accusing them of being bad.
In other words, the check engine light is back on, and you're still getting O2 sensor codes.
When this happens, it's usually because the engine is running either too rich or too lean —but the real cause isn't the oxygen sensor itself.
In plain English: some other component is throwing off the air/fuel mixture, and that's what's making the O2 sensor look guilty.
This could be caused by:
- Failing fuel pump: Can't deliver enough fuel pressure, causing the engine to run lean and setting off O2 sensor codes.
- Leaking intake manifold gaskets: Allow unmetered air into the engine, creating a lean condition the PCM can't compensate for.
- Clogged or dirty fuel injectors: Starve individual cylinders of fuel, leading to lean misfires and confusing the O2 sensors.
- Bad MAP sensor (Manifold Absolute Pressure): Sends incorrect load data to the PCM, which can cause rich or lean fuel trim issues.
- Faulty ECT sensor (Engine Coolant Temperature): If it reads colder than it should, the PCM will stay in open loop too long and over-fuel the engine.
- Bad IAT sensor (Intake Air Temperature): If it's way off, the PCM may think the air is colder or hotter than it really is, messing with the mixture.
- Thermostat stuck open or wrong temperature: Keeps the engine running too cool, making the PCM run a richer mixture longer than it should.
- Exhaust leaks before the O2 sensor: Let ambient air into the exhaust stream, tricking the O2 sensor into thinking the engine is running lean when it's not.
All of these can trick the PCM into over-fueling or under-fueling the engine —which in turn causes the O2 sensor to report a constant rich or lean condition, making it look like the sensor is the problem.
The frustrating part? The real issue isn't always present when you're testing. This kind of intermittent problem is common —especially when sensors or gaskets are just starting to go bad.
Here's the best strategy: wait it out a few days. Let the problem get a little worse. When it becomes more consistent, it'll be easier to catch with your scan tool —and that's when you'll be able to test it, confirm it, and finally fix it.
More 5.2L, 5.9L V8 Dodge Ram Pickup Test Articles
I've written quite a few 5.2L, 5.9L V8 Dodge Ram pickup 'how to' tutorials. You can find all here:
If this info saved the day, buy me a beer!
