How to Test Trouble Code P0420
(GM 3.8L)

How to Test a P0420 Diagnostic Trouble Code (GM 3.8L)

OBD II trouble code P0420 Catalyst Efficiency Below Threshold (Bank 1) is one of the most common codes encountered on the GM 3.8L equipped cars and mini-vans.

In the majority of the cases, this code points to a bad catalytic converter. In this tutorial, I'm gonna' show you some of the basics you need to know about what DTC P0420 means and a step-by-step test on how to test it.

Here are the contents of this tutorial at a glance:

  1. P0420 Basics You Need to Know.
  2. Symptoms of a P0420 Diagnostic Trouble Code.
  3. What Causes the Catalytic Converter to Fail.
  4. How to Diagnose Trouble Code P0420: Catalyst Efficiency Below Threshold (Bank 1).
  5. TEST 1: Checking for a Broken Catalytic Converter.
  6. TEST 2: Checking the Output of the Rear Oxygen Sensor.
  7. TEST 3: Manually Inducing a Lean Condition.
  8. TEST 4: Manually Inducing a Rich Condition.
  9. Where to Buy the O2 Sensor and Save $$$.
  10. More Test Articles.

P0420 Basics You Need to Know

One of the most important things we need to know is that code P0420 is referring to the performance of the catalytic converter on your 3.8L V-6 equipped GM car or mini-van. Specifically, that the catalytic converter is not cleaning up the exhaust of hydrocarbons like it should.

In case you're wondering what the heck are hydrocarbons?... in a nutshell, and as it applies to automobiles, this word refers to gasoline... specifically the unburned gasoline that flows into the exhaust after the combustion process.

As you're probably already aware, the catalytic converter is tasked with cleaning as much of the hydrocarbons, that don't get burned in the cylinder during the combustion process, as it can. Although I'm not going into minute technical detail, on how the cat works, here are some more specifics about what it does:

  1. In a normally running engine, gasoline doesn't get burned completely when it's ignited by the spark plug's spark (inside the cylinder) and although this is not the ideal thing to happen, it's completely normal.
  2. These hydrocarbons (unburned and partially burned fuel) pass into the exhaust where they are funneled into the catalytic converter.
  3. Once in the catalytic converter, these hydrocarbons are oxidized into their basic components... which are carbon dioxide and water.
  4. In addition to converting hydrocarbons into something less harmful, the catalytic converter also oxidizes:
    1. Nitrogen oxides (2NOx) to nitrogen and oxygen.
    2. Carbon monoxide to carbon dioxide.
  5. An ignition system misfire or any other fuel control system malfunction, that causes the fuel injection computer to inject excess fuel (due to a loss of fuel trim control), will cause the catalytic converter to overheat and eventually malfunction... and set a P0420 DTC.

If you've ever noticed water dripping out of the exhaust of a running car... then you're seeing the end result of the oxidation process happening inside the catalytic converter.

Symptoms of a P0420 Diagnostic Trouble Code

The symptoms (or lack of symptoms) of a BAD catalytic converter depend on the way the catalytic converter has failed. I'll explain...

...There are two specific ways the catalytic converter fails. One is where the honeycomb filter element inside the catalytic converter simply wears out and can no longer clean the exhaust but, remains intact (in one piece). The other is when the honeycomb filter element shatters into several pieces.

Depending on how the catalytic converter fails... you'll see the following:

  1. The Catalytic Converter Filter Element HAS NOT Broken Into Pieces:
    1. Check engine light (CEL) is on.
    2. DTC P0420 is present.
    3. ‘Rotten egg’ smell coming from the exhaust.
    4. Won't pass the state mandated emissions test.
    5. Gas mileage will suffer.
    6. There are no symptoms like: lack of power, rough idle, or hesitation.
  2. The Catalytic Converter Filter Element has Broken Into Pieces:
    1. Lack of power (caused by broken pieces blocking/restricting the exhaust flow).
    2. Hesitation when accelerating the vehicle (caused by broken pieces blocking/restricting the exhaust flow).
    3. Rough idle (caused by broken pieces blocking/restricting the exhaust flow).
    4. Rattle coming from underneath vehicle when accelerating.
    5. Check engine light (CEL) is on.
    6. DTC P0420 is present.

Let's take a look at the next subheading and find out why the catalytic converter fails.

What Causes the Catalytic Converter to Fail

The catalytic converter will eventually fail all on its own... but what accelerates its demise are several things:

  1. An excessively Rich condition. This usually caused by one of the following:
    1. Ignition system misfire.
    2. Leaking fuel injector(s).
    3. Leaking fuel pressure regulator.
    4. EVAP canister leaking fuel into the intake manifold.
  2. Antifreeze entering the exhaust. This is usually caused by one of the following:
    1. blown head gasket.
    2. coolant leaking into the cylinder(s) from bad intake manifold gaskets. The gaskets on the OBD II equipped 3.8L Engines are made of plastic and are prone to leaking coolant.
  3. Engine oil.
    1. Engine oil, burning inside the cylinder from worn out piston rings, coating the reactive surfaces of the cat, reducing its performance.
    2. Engine oil, burning inside the cylinder from leaking valve stem seals, coating the reactive surfaces of the cat, reducing its performance.
    3. Adding engine oil anti-wear additives that contain zinc.

What's a Rich condition? This is when the engine is consuming an excessive amount of fuel. This excessive amount of fuel, entering the engine, causes raw fuel (hydrocarbons) to pass into the exhaust and into the catalytic converter. Once in the catalytic converter, this excessive amount of fuel causes the catalytic converter to overheat and fail.

Knowing that your vehicle has any one of the above conditions/problems, will help you to make an informed decision as to what needs to get repaired to successfully solve the P0420 Catalyst Efficiency Below Threshold (Bank 1) trouble code.

Alright, let's turn the page and get testing!.....

Buick Vehicles:

  • Century 3.8L
    • 2005
  • LeSabre 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Park Avenue (& Ultra) 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

Buick Vehicles:

  • Regal 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004
  • Riviera 3.8L
    • 1995, 1996, 1997, 1998, 1999

Chevrolet Vehicles:

  • Camaro 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002

Chevrolet Vehicles:

  • Impala 3.8L
    • 2000, 2001, 2002, 2003, 2004, 2005
  • Lumina (& MPV) 3.8L
    • 1995, 1998, 1999
  • Monte Carlo 3.8L
    • 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

Oldsmobile Vehicles:

  • 88 (& 88 Royale) 3.8L
    • 1995, 1996, 1997, 1998, 1999
  • 98 Regency 3.8L
    • 1995, 1996, 1997, 1998
  • Intrigue 3.8L
    • 1998, 1999
  • LSS 3.8L
    • 1996, 1997, 1998, 1999
  • Regency 3.8L
    • 1997, 1998

Oldsmobile Vehicles:

  • Silhouette 3.8L
    • 1995

Pontiac Vehicles:

  • Bonneville 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Firebird 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002

Pontiac Vehicles:

  • Grand Prix 3.8L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008
  • Trans Sport 3.8L
    • 1995

“I went to a bookstore and asked the saleswoman, ‘Where’s the self-help section?’
She said if she told me, it would defeat the purpose.”

George Carlin

MORE TUTORIALS: