What Is O2S11 And O2S21?
On your scan tool (when it's in Live Data mode), the pre catalytic converter oxygen sensors are identified as O2S11 and O2S21.
O2S11 is shorthand for Oxygen Sensor Bank 1 Sensor 1. On the 5.0L and 5.7L V8 engines, Bank 1 is the engine bank that houses cylinders 1, 3, 5, and 7. On the 4.3L V6 engine, Bank 1 is the one that houses cylinders 1, 3, and 5.
O2S21 is shorthand for Oxygen Sensor Bank 2 Sensor 1. On the 5.0L and 5.7L V8's, Bank 2 is the one that contains the cylinders 2, 4, 6, and 8. On the 4.3L V6 engine, Bank 2 is the one that has cylinders 2, 4, and 6.
The part that refers to them as Sensor 1 simply means that they are located before the catalytic converter. When the O2 sensors are located behind the catalytic converter, they are referred to as Sensor 2... as in O2S12 and O2S22.
O2 SENSOR TEST 1
To test the oxygen sensors, the very first thing you'll do is to induce a Rich Condition.
You'll manually create this Rich Condition by spraying a little carburetor cleaner into an available vacuum hose while the engine is running.
If the O2 sensors are good, they'll both produce 0.900 Volts on the scan tool once the carb spray enters in the engine.
Alright, this is what you'll need to do:
Connect your scan tool to the diagnostic connector under the dash and start the engine (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
The engine must run for about 15 minutes to get the O2 sensors to activate.
Scroll down to the PID's that are labeled O2S11 and O2S21. These are the PID's that will show you the voltages the O2 sensors are creating.
Now, take a look at the voltage readings for O2S11 and O2S21.
If the engine has been running for about 15 minutes, and the 02 sensors are OK, you should see the voltages moving between 0.200 Volts and 0.900 Volts.
If the Voltages are not moving between 0.100 and 0.900 Volts, don't worry about it just yet... continue on to the next step.
With the engine running, spray a little carburetor cleaner into a vacuum hose (that has engine vacuum) while you observe your scan tool's display screen.
You're not going to be able to spray carb cleaner into the throttle as the engine is running, because if you were to do this, the engine will die as soon as you disconnect the air duct from it (to spray into it).
If you spray too much, the engine will stall. If this happens to you, just restart the engine and repeat the step and spray less carb cleaner spray.
As you spray some short burst of carb cleaner into the vacuum hose, you should see the voltage numbers of O2S11 and O2S21 immediately spike to 0.800 to 0.900 Volts. And as long as you're spraying, these voltage number should stay there.
When you stop spraying, the O2 sensors values should come down and within a few seconds, they should start oscillating between 0.100 Volts to 0.900 Volts. If they don't... don't worry about yet.
OK, the test is done, let's take a look at what your test results mean:
TEST RESULT 1: O2S11 and O2S21 voltage numbers spiked to 0.900 Volts when you sprayed carb spray into the vacuum hose -This tells you that both oxygen sensors are OK at this point in time. They do not need to be replaced, since what is causing the PCM to think they are fried is something else.
For more info on this, go to the section: Oxygen Sensor Codes Keep Coming Back.
TEST RESULT 2: O2S11 voltage numbers DID NOT spike to 0.900 Volts when you sprayed carb spray into the vacuum hose -This confirms that Bank 1 Oxygen Sensor 1 is no longer working. You can replace the oxygen sensor.
TEST RESULT 3: O2S21 voltage numbers DID NOT spike to 0.900 Volts when you sprayed carb spray into the vacuum hose -This confirms that Bank 2 Oxygen Sensor 1 is no longer working. You can replace the oxygen sensor.