How To Diagnose Trouble Code P0420: Catalyst Efficiency Below Threshold (Bank 1)
START HERE: Although in about 90% of the time, seeing a P0420 register on your scan tool indicates that the catalytic converter has failed, you should still verify that the cat indeed has failed. Why?
Because checking to see if the catalytic converter is bad on your 3.8L equipped car or mini-van involves 3 to 4 simple tests. In a nutshell, you'll need to:
- Check that the catalytic converter's filter element hasn't shattered into pieces.
- This involves physically banging on the catalytic converter to see if you can hear anything rattling inside.
- TEST 1: Checking For A Broken Catalytic Converter.
- Checking to see if the rear oxygen sensor is seeing a Rich condition
- This involves checking the rear oxygen sensor's output voltage, with your scan tool in Live Data mode, to see if it's above 0.5 Volts.
- TEST 2: Checking The Output Of The Rear Oxygen Sensor.
- Induce a Lean condition to see if the downstream oxygen sensor reports it.
- This involves checking to see if the rear oxygen sensor can report a Lean condition that you'll manually create (by unplugging a large vacuum hose).
- TEST 3: Manually Inducing A Lean Condition.
- Induce a Rich condition to see if the downstream oxygen sensor reports it.
- This involves checking to see if the rear oxygen sensor can report a Rich condition that you'll manually create (by spraying a small amount of carburetor cleaner into a small vacuum hose).
- TEST 4: Manually Inducing A Rich Condition.
You'll need a scan tool that has Live Data capability to perform TEST 2 and TEST 3. OK, let's get started!
TEST 1: Checking For A Broken Catalytic Converter
The very first thing we need to do is to see if the catalytic converter's filter element (this is the one that's inside and does all of the exhaust gas cleaning) has shattered into pieces or not.
If the cat's filter element has broken into several pieces, you'll be able to hear a rattle as you bang on the catalytic converter with the palm of your hand.
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
Jack up your vehicle 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
Bang on the catalytic converter.
I have always used the palm of my hand to do this, you can too. - 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 3.8L equipped car or mini-can is not passing excessive unburned fuel (hydrocarbons). Because if it is, then 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.
