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

START HERE: You can verify the state of the catalytic converter to see if it has fried or not. The tests I'm gonna' show you are not hard and before we jump right into them, I'll briefly go over them in this section.

  1. Check to see if the filter element is broken inside the catalytic converter.
    1. This is a very simple and fast test.
    2. TEST 1: Checking For a Broken Catalytic Converter.
  2. Confirming that the downstream oxygen sensor is seeing a Rich condition.
    1. This simply involves checking the voltage that the rear O2 sensor is reporting.
    2. Specifically, you need to see if this voltage is stuck above 0.5 Volts DC (500 millivolts), since this would indicate the catalytic converter is not scrubbing clean the exhaust of hydrocarbons.
    3. Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 Scan Tool Review.
    4. TEST 2: Checking The Output Of The Rear Oxygen Sensor.
  3. Induce a Lean condition to see if the downstream oxygen sensor reports it.
    1. With this test, we'll check the performance of the rear oxygen sensor and make sure it's working correctly (since there's a good chance it has failed and thus lying about the cat being bad).
    2. This is accomplished by unplugging a large vacuum hose and checking to see if the downstream oxygen sensor reports the vacuum leak.
    3. You'll need to use your scan tool in Live Data mode.
    4. TEST 3: Manually Inducing A Lean Condition.
  4. Induce a Rich Condition to see if the downstream oxygen sensor reports it.
    1. In some cases, the rear oxygen sensor reports a normally functioning catalytic converter (although it has failed)... so with this test, you'll test the performance of the rear O2 sensor.
    2. 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.
    3. You'll need to use your scan tool in Live Data mode.
    4. 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 (Honda 2.2L, 2.3L)

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

As I mentioned earlier, it's rare for the catalytic converter's filter element to shatter in a Honda but it does happen. Since checking to see if the cat's filter element is broken or not is a very easy test, it's worth doing and thus eliminate this possibility.

In a nutshell, you'll bang on the catalytic converter, with the palm of your hand, and listen for any rattles (coming from inside the converter).

IMPORTANT: Perform this test on a completely cold engine and catalytic converter! The catalytic converter retains heat long after the engine has been shut off. 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:

  1. No ignition system misfires that are allowing excessive unburned fuel into the exhaust.
  2. No intake manifold gasket leaks that could allow anti-freeze into the engine's cylinders.
  3. A malfunctioning fuel injectin 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 Honda 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.

Honda Vehicles:

  • Accord 2.2L, 2.3L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • Odyssey (EX LX) 2.2L
    • 1995, 1996, 1997, 1998
  • Prelude 2.2L
    • 1995, 1996

Acura Vehicles:

  • CL 2.2L
    • 1997, 1998, 1999

Isuzu Vehicles:

  • Oasis 2.2L
    • 1996, 1997, 1998, 1999