TEST 2: Checking The Output Of The Rear Oxygen Sensor
When the catalytic converter stops scrubbing the exhaust of hydrocarbons, the downstream oxygen sensor will show a continuous/fixed voltage above 0.5 Volts. In other words, it's gonna' show a chronic Rich condition.
You and I can confirm this by using a scan tool with Live Data capability and observing the voltage of the PID labeled O2S12.
In the photo above, the arrow points to this PID. In case you're wondering, O2S12 refers to the downstream oxygen sensor. (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 Scan Tool Review).
Alright, let's get started:
- 1
Connect your scan tool to your car or mini-van and start the engine (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 Scan Tool Review).
Let the engine idle for at least 15 minutes before you start the test, to get the catalytic converter to activate. - 2
On your scan tool (and once you're in Live Data mode), scroll down to the PID that's labeled O2S12. This PID is the one that will show you what the rear oxygen sensor is reporting in Volts DC.
- 3
Now, take a look at the voltage readings for O2S12.
When the catalytic converter is failing, you'll see a voltage that stays above 0.5 Volts DC (in a normally operating catalytic converter, the rear O2 sensor should report a steady voltage between 0.100 Volts and 0.500 Volts).
NOTE: Unlike the front O2 sensor (O2S11), whose voltage varies constantly/rapidly between 0.1 to 0.9 Volts, the rear O2 sensor's voltage should fluctuate less and stay around 0.5 Volts or less.
What is the oxygen sensor reporting on your scan tool?
CASE 1: The downstream O2 sensor's voltage fixed above 0.5 Volts. This usually confirms that the catalytic converter has failed but not always.
There's a good chance that the rear oxygen sensor has failed and is stuck reporting a fixed high voltage to the PCM. So the next step is to induce a Lean condition and see if the rear oxygen sensor reacts and reports the change. For this test, go to: TEST 3: Manually Inducing A Lean Condition.
CASE 2: The downstream O2 sensor's voltage IS NOT fixed above 0.5 Volts. This usually means that the catalytic converter is failing intermittently.
But there's also a good chance that the rear oxygen sensor is bad and need to be replaced. The good thing is that you can test the performance of the rear O2 sensor to see if good or bad. For this test, go to: TEST 4: Manually Inducing a Rich Condition.
TEST 3: Manually Inducing A Lean Condition
There's a good possibility that the downstream O2 sensor has failed and is stuck producing a high voltage above 0.5 Volts DC.
This could lead you, I, and the PCM to think that the catalytic converter has failed when it really hasn't.
So, in this section, you'll test the performance of the rear O2 sensor (O2S12) to see if it's really reporting the truth (about this Rich condition).
OK, this is what you'll need to do:
- 1
With your scan tool still in Live Data mode (and the engine running), scroll down to the PID that's labeled O2S12. This PID is the one that will show you what the rear oxygen sensor is reporting in Volts DC.
- 2
With the engine running, have a helper slightly disconnect a big vacuum hose while you observe your scan tool's O2S12 voltage values.
NOTE: The idea here is to NOT completely disconnect this vacuum hose while engine is running or the engine may stall. The idea is let a sufficient amount of air to Lean out the air/fuel mixture. - 3
As ambient air starts to get sucked into the vacuum hose's port, you should see the voltage numbers of O2S12 immediately go down to about 0.100 Volts.
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Reconnecting the vacuum hose and see if the rear O2 sensor's reading goes back to it previous activity.
Let's take a look at what your test results mean:
CASE 1: The downstream O2 sensor's voltage decreased to 0.1 Volt. This confirms that the downstream oxygen sensor is OK and is able to fully respond to the changes in the air/fuel mixture.
This also means that the high voltage the downstream O2 sensor is reporting is being caused by a genuine Rich condition and the catalytic converter has failed.
Before you replace the catalytic converter, you need to make sure that your car or mini-van doesn't have any other fuel or ignition system malfunction that may be causing unburned fuel to pass into the exhaust. Here are some suggestions:
- Check fuel pressure, with a fuel pressure gauge, to see if it's too high.
- Check for leaking fuel injectors.
- EVAP canister vacuum hose leaking fuel into the intake manifold.
- A throttle position sensor (TPS) that is failing intermittently.
- Make sure all tune up parts (spark plugs, spark plug wires, distributor cap, etc.) don't have excessive wear and tear.
- Check for low engine compression.
CASE 2: The downstream O2 sensor's voltage DID NOT decrease to 0.1 Volt. This tells you that the downstream oxygen sensor is bad and needs to be replaced.
If the oxygen sensor was operating normally, it would have reacted to the removal of the PCV Valve (from the valve cover) immediately by producing a voltage around 0.1 Volts (100 millivolts) since it did not, you can deduce correctly that it's fried and needs to be replaced.
CASE 3: The downstream O2 sensor's wiring is melted to the exhaust pipe. Replace the downstream oxygen sensor.