TEST 4: Manually Inducing a Rich Condition

How To Test A P0420 Diagnostic Trouble Code (GM 3.1L, 3.4L)

Usually, when a P0420 lights up the check engine light... the catalytic converter has failed and can no longer oxidize those unburned hydrocarbons passing into the exhaust but not always.

In some cases, the catalytic converter is just starting to fail, so when you test/check its performance, the rear oxygen sensor reports a catalytic converter that seems to be working just fine.

To be more specific, the rear oxygen sensor does not report continuous high voltage around .9 volts or more.

When this happens, you and I need to test the performance of the rear oxygen sensor to make sure that it's not lying to us. We can check the performance of the rear oxygen sensor, to see if it's good or not, by inducing a Rich condition (with carburetor spray).

This is a very simple test and I'll show you how to do it and interpret its results.

This is what you'll need to do:

  1. 1

    Connect your scan tool to the 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 .1 to .9 Volts, the rear O2 sensor's voltage should fluctuates less and stays around .5 Volts or less.

  4. 4

    With the engine running, have a helper disconnect the vacuum hose that attaches to the fuel pressure regulator and spray a small shot of carburetor spray.

    The engine may stall, if your helper sprays too much, if this happens just restart the engine.

  5. 5

    As the carb spray enters the vacuum hose... you should see the voltage numbers of O2S12 immediately go up to about 0.900 Volts.

  6. 6

    A few seconds after your helper has stopped spraying carb. cleaner, the rear oxygen sensor voltage numbers should go back down.

Let's take a look at what your test results mean:


CASE 1: The downstream O2 sensor's voltage shot up to .9 Volts This test result tells you that the rear O2 sensor (O2S12) is working.

Since the rear oxygen sensor is working properly, you now know that the catalytic converter is starting to fail, although intermittently. Your next step is to make sure that your engine isn't suffering an abnormal Rich condition. You should:

  1. Check for and diagnose/repair any trouble codes lighting up the check engine light (CEL).
  2. Check fuel pressure, with a fuel pressure gauge, to see if it's too high.
  3. Check for leaking fuel injectors.
  4. EVAP canister vacuum hose leaking fuel into the intake manifold.
  5. Make sure all tune up parts (spark plugs, spark plug wires, distributor cap, etc.) don't have excessive wear and tear.
  6. Check for low engine compression.

CASE 2: The downstream O2 sensor's voltage DID NOT shoot up to .9 Volts 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 addition of the carburetor spray immediately by producing a voltage around .9 Volts (900 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.