How To Test Trouble Code P0420 (GM 3.8L)

TEST 2: Checking The Output Of The Rear Oxygen Sensor

In this test section, I'm gonna' show you how to check to see if the rear oxygen (O2) sensor is reporting a continuous high voltage above 0.5 Volts (500 millivolts).

If the rear O2 sensor is reporting a fixed high voltage, then this would indicate a Rich condition, which in turn would point to a catalytic converter that's not cleaning up the unburned hydrocarbons in the exhaust or a bad rear oxygen sensor.

You'll need a scan tool has to have Live Data capability so you can see the actual voltage the rear oxygen sensor is producing/reporting (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. 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. 2

   Now, 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.
   
   NOTE: It's the rear oxygen sensor (O2S12) that measures the activity of the catalytic converter.

3. 3

   Now, take a look at the voltage readings for O2S12.
   
   The voltage reported should be 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?

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

In TEST 2 you verified that the rear oxygen sensor is indeed sensing and reporting a Rich condition. As you already know, this Rich condition is usually a result of a catalytic converter that can no longer oxidize the unburned hydrocarbons that are passing through it.

Now, you'll test the performance of the rear O2 sensor (O2S12) to see if it's really reporting the truth (about this Rich condition). Why? Because oxygen sensors are known to fail internally and produce a fixed high voltage that fools the PCM into thinking the catalytic converter has failed.

OK, this is what you'll need to do:

1. 1

   With your scan tool still in Live Data mode and the engine running from the previous test, 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. 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. See photo above.

3. 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.

4. 4

   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 responding 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. Your next steps are to see why excessive fuel is being dumped into the cylinders (and thus causing a Rich condition). 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.