How To Test: P0420 OBD II Trouble Code (GM 3.1L, 3.4L)

How To Diagnose Trouble Code P0420 Catalyst Efficiency Below Threshold (Bank 1)

START HERE: Checking to see if the catalytic converter has actually failed isn't hard and it's something that the DIY'er can do at home.

I'm gonna' show you 4 simple tests that you can do to check the catalytic converter and find out if it's bad. In a nutshell, you'll need to:

  1. Check that the catalytic converter's filter element hasn't shattered into pieces.
    • All that needs to be done, to check to see if the catalytic converter's filter element has shattered, is to bang on it with the palm of your hand. If it's busted inside, you'll hear the pieces rattling inside.
    • NOTE: This test has to be done with a completely cold engine!
    • TEST 1: Checking For A Broken Catalytic Converter.
  2. Verifying that the downstream oxygen sensor is seeing a Rich condition.
  3. Induce a Lean condition to see if the downstream oxygen sensor reports it.
    • This is accomplished by unplugging a large vacuum hose and checking to see if the downstream oxygen sensor reports the vacuum leak.
    • You'll need to use your scan tool in Live Data mode.
    • TEST 3: Manually Inducing A Lean Condition.
  4. Induce a Rich Condition to see if the downstream oxygen sensor reports it.
    • This accomplished by spraying a small amount of carburetor cleaner into a small vacuum hose, usually the fuel pressure regulator's vacuum hose/line, and checking to see if the rear O2 sensor reports the addition of the carb. spray.
    • You'll need to use your scan tool in Live Data mode.
    • TEST 4: Manually Inducing a Rich Condition.

I recommend that you start with TEST 1: Checking For A Broken Catalytic Converter, but you can modify the following tests to fit your particular diagnostic needs.

TEST 1: Checking For A Broken Catalytic Converter

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

To get started, we'll check to see if the catalytic converter's filter element has shattered into pieces or not.

This is a pretty simple test that requires that you bang on the catalytic converter, with the palm of your hand, and listen for any rattles (coming from inside the converter).

IMPORTANT: The engine, and thus the catalytic converter, must be completely cold to perform this test! Be careful, take all necessary safety precautions, and use common sense to avoid burns on your hands. Your safety is your responsibility.

This is what you need to do:

  1. 1

    Jack up your car or mini-van and place it on jack stands. The catalytic converter is only accessible from underneath vehicle.

    IMPORTANT: Use jack stands to hold up the car or mini-van up in the air. Don't trust the jack alone to hold up the vehicle!

    IMPORTANT: Your vehicle's engine MUST BE completely cold to do this test.

  2. 2

    Bang on the catalytic converter.

    I have always used the palm of my hand to do this, you can too.

  3. 3

    Listen to see if there's a rattle coming from inside the catalytic converter.

    If the catalytic converter's element is broken into pieces, you'll hear a definite rattle when banging on it with the palm of your hand.

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

CASE 1: The catalytic converter rattled. This confirms that the catalytic converter's filter element has broken into pieces and needs to be replaced.

Before replacing the catalytic converter, you need to make sure that:

  • No ignition system misfires that are allowing excessive unburned fuel into the exhaust.
  • No intake manifold gasket leaks that could allow anti-freeze into the engine's cylinders.
  • A malfunctioning fuel injection sensor that could be causing a Rich condition and thus allowing excessive unburned fuel into the exhaust.

The idea is to make sure that the engine in your 3.1L/3.4L equipped car or mini-can is not passing excessive unburned fuel (hydrocarbons). Because if it is, you'll need to repair this issue before replacing the catalytic converter (or you'll a repeat of the problem).

CASE 2: The catalytic converter DID NOT rattle. This confirms that the catalytic converter's filter element is not broken into pieces.

The next step is to check the performance of the catalytic converter by checking the voltage values of the rear oxygen sensor and manually inducing a Lean or a Rich condition. Go to: TEST 2: Checking The Output Of The Rear Oxygen Sensor.