P0420 -What Does It Mean? (1996-1999 3.8L V6 Buick, Chevrolet, Pontiac, Olds)

Catalytic converters don't last forever and when they fail, the fuel injection computer is going to light up the check engine light with a P0420: Catalyst System Efficiency Below Threshold diagnostic trouble code.

In this article, I'll explain what causes the catalytic converter to fail and what you should do before replacing it (to avoid a repeat of the problem).

You can find this tutorial in Spanish here: Código P0420 ¿Qué Significa? (1996-1999 3.8L V6 Buick, Chevrolet, Pontiac, Olds) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 3.8L Buick LeSabre: 1996, 1997, 1998, 1999.
• 3.8L Buick Park Avenue: 1996, 1997, 1998, 1999.
• 3.8L Buick Regal: 1996, 1997, 1998, 1999.
• 3.8L Buick Riviera: 1996, 1997, 1998, 1999.

• 3.8L Chevrolet Lumina: 1998, 1999.
• 3.8L Chevrolet Monte Carlo: 1998, 1999.

• 3.8L Oldsmobile 88 (Eighty-Eight): 1996, 1997, 1998, 1999
• 3.8L Oldsmobile 98 (Ninety-Eight): 1996
• 3.8L Oldsmobile Intrigue: 1998, 1999
• 3.8L Oldsmobile LSS: 1996, 1997, 1998, 1999
• 3.8L Oldsmobile Regency: 1997, 1998

• 3.8L Pontiac Bonneville: 1996, 1997, 1998, 1999
• 3.8L Pontiac Grand Prix: 1997, 1998, 1999

What Does The Catalytic Converter Do?

As the engine runs, in your 3.8L V6 equipped GM vehicle, it is creating and emitting three specific types of gases that are harmful to the environment.

These three harmful gases are hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).

The catalytic converter has the job of reducing and converting them into harmless gases.

Unfortunately, the catalytic converter will not last forever and when it starts to fail, it isn't able to reduce the harmful pollutants that the engine is creating and emitting.

Since the catalytic converter is constantly monitored by the fuel injection computer, when it fails the computer will set a P0420 and illuminates the check engine light.

Common Symptoms Of A P0420 Trouble Code

You're gonna' see one or more of the following symptoms:

• The check engine light is on.
• If catalytic converter has broken into pieces, you'll notice a lack of power when you accelerate the engine under load.
• Bad gas mileage.
• Rotten egg or sulfur smell coming out of the tailpipe when the engine is running (especially when you accelerate the engine under load).

What Causes A P0420 Trouble Code?

It's a very common misconception that a bad oxygen sensor can cause a P0420 trouble code (although a bad one can contribute to the catalytic converter's demise).

The only component that can set a P0420 trouble code is a bad catalytic converter.

The most common cause of a bad catalytic converter, is too much unburned fuel (hydrocarbons) entering the exhaust; which will overload, overheat, and eventually destroy the catalytic converter's ability to reduce contaminants.

Troubleshooting a P0420 trouble code involves checking if one of the following issues is present:

1. An excessively rich air/fuel mixture that is overloading and overheating the catalytic converter. An excessively rich air/fuel mixture is usually the result of an:
   • Ignition system misfire.
   • Leaking fuel injector(s).
   • Leaking fuel pressure regulator.
   • EVAP emission canister leaking fuel into the intake manifold when it shouldn't.
2. Antifreeze entering the exhaust. This is usually caused by one of the following:
   • Intake manifold gaskets leaking coolant (a very, very common problem in the 3.8L GM V6 engine).
   • Blown head gaskets.
3. The engine is burning engine oil.
   • Engine oil, burning inside the cylinder from worn out piston rings, will coat the reactive surfaces of the catalytic converter, reducing its performance.
   • Engine oil, burning inside the cylinder from leaking valve stem seals, coating the reactive surfaces of the catalytic converter, reducing its performance.
   • Adding engine oil anti-wear additives that contain zinc.

Before You Replace The Catalytic Converter

To solve the P0420 trouble code, you'll need to replace the catalytic converter, but before you do, it's important that you first address what made it fail.

I recommend that you troubleshoot and resolve any problem that could be:

• Overloading the exhaust with hydrocarbons (raw gasoline).
• Causing anti-freeze to enter the exhaust gas.
• Causing engine oil to enter and burn in the engine cylinders.

In my experience, the most common cause of a bad catalytic converter has been a misfire problem.

The other issue I've encountered, is intake manifold gaskets leaking coolant into the cylinders.

Resolve any misfire trouble codes (P0300, P0301, P0302, P0303, P0304), if you have them, first.

Check to see if the intake manifold gaskets are leaking coolant, if they are, replace them before replacing the catalytic converter.

Also, if any other trouble codes are present along with the P0420 trouble code (especially the ones that indicate a problem with the air/fuel mixture), troubleshoot and resolve them first before replacing the catalytic converter.

If you don't resolve any of these problems (if they're present), then the new catalytic converter will also fail and you're back to square one with a P0420 lighting up the check engine light.

More 3.8L GM V6 Tutorials

You can find a complete list of 3.8L GM V6 tutorials in this index:

• GM 3.8L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

• How To Test The MAF Sensor (GM 3.8L V6 1996-2005).
• How To Test A Does Not Crank Condition -Case Study (GM 3.8L).
• GM 3.8L Ignition Control Module And Crank (3X, 18X) Sensor Test.
• How To Test The Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L).