P0420 Diagnostics -Catalytic Converter Tests (1996-2000 3.0L V6 Caravan, Grand Caravan, Voyager, Grand Voyager)

If your 1996–2000 3.0L V6 Chrysler, Dodge, or Plymouth minivan has a check engine light with code P0420, the catalytic converter isn't doing its job anymore.

In this tutorial, I'll walk you through the most common reasons the cat fails —and if you're up for it— a few simple, hands-on tests you can do to check the catalytic converter and the performance of the rear oxygen sensor (the one that monitors the cat).

You don't need any expensive tools, just a scan tool with live data capability.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.0L V6 Dodge Caravan: 1996, 1997, 1998, 1999, 2000.
• 3.0L V6 Dodge Grand Caravan: 1996, 1997, 1998, 1999, 2000.
• 3.0L V6 Plymouth Voyager: 1996, 1997, 1998, 1999, 2000.
• 3.0L V6 Plymouth Grand Voyager: 1996, 1997, 1998, 1999, 2000.

Common Signs of a Failing Catalytic Converter

On your 3.0L V6 Chrysler, Dodge, or Plymouth minivan, a failing catalytic converter usually (but not always) won't cause any obvious or disruptive symptoms at first.

The two most common failures are:

• The catalytic converter's ceramic core simply stops doing its job of filtering out harmful emissions. No noticeable impact on engine performance, just a P0420 code lighting up the check engine light.
• The internal ceramic element breaks apart and partially blocks the exhaust flow —you'll definitely notice engine performance taking a big hit.

Either scenario is not good. Here's a closer look at the most common signs of a bad or failing catalytic converter:

• Check engine light with code P0420: The PCM compares data from the front and rear oxygen sensors to monitor converter efficiency. If it detects that the converter isn't reducing emissions properly, it sets code P0420.
• When this code is stored, the converter is no longer doing its job.
• Loss of power under load: If the converter's ceramic core shatters into pieces, it'll partially clog the exhaust flow. This builds up backpressure, making the engine feel sluggish —especially during acceleration or hill climbs.
• Bad gas mileage: A blocked or restricted converter forces the engine to work harder to push exhaust gases out, you'll definitely be filling up more often.
• Rattling noise underneath the vehicle: When the cat's ceramic core breaks apart, it often makes a knocking or rattling sound. You'll usually hear it at startup or when revving the engine.
• Strong sulfur or "rotten egg" smell: A bad or failing converter may emit a foul smell —especially when the engine is under load or running rich.
• This is probably the second most common symptom of a bad catalytic converter —right behind the P0420 trouble code.
• Visible overheating of the converter: A bad or overworked converter may glow red-hot. This is most noticeable at night as you'll see a red glow underneath your minivan.

What Causes Catalytic Converter Failure?

The catalytic converter on your minivan is built to last —but not forever. Over time, certain engine and fuel system problems can take a toll and gradually ruin its ability to clean up exhaust gases.

Below are the most common culprits behind catalytic converter failure:

• Misfiring cylinders: When a cylinder misfires, unburned fuel gets pushed into the exhaust stream. That raw fuel can ignite inside the converter or overwork it, spiking it temperature and damaging its ceramic core.
• Oil or coolant entering the combustion chamber: Worn piston rings or a bad head gasket can allow oil or coolant to seep in and burn. The byproducts coat the converter's internal surfaces, reducing its ability to filter harmful emissions.
• On high-mileage engines, this is one of the most frequent causes of cat failure.
• Leaking fuel injectors: Leaky injectors lead to excess fuel being delivered to the engine —fuel that doesn't burn completely and ends up in the exhaust. This overworks and overheats the converter, wearing it down over time.
• Sensor-related fueling issues: A bad O2 sensor, MAP sensor, or coolant temp sensor (to name a few) can throw off the PCM's fuel calculations.
• If the faulty sensor issue isn't fixed soon enough, the resulting rich air/fuel mixture can overload the cat, overheat it, and damage it over time.
• Cool-running engine from a stuck-open thermostat: If the thermostat doesn't close properly, the engine never reaches full operating temp.
• The PCM compensates by adding more fuel, which again results in excessive unburned fuel entering the converter and damaging it.
• Physical impact damage: Being located underneath the vehicle puts the converter at risk from debris, speed bumps, or potholes. A hard impact can crack or shatter its internal core.

Keep in mind —replacing a failed converter without correcting the root cause is just a short-term fix.

If you don't resolve the underlying issue, you'll end up destroying the new one too. So before installing a replacement, make sure you've identified and addressed what caused the original to fail.

TEST 1: Checking For A Broken Catalytic Converter

One of the most common failures a catalytic converter can experience is internal breakage.

In this first test, we'll inspect for physical damage —specifically a broken ceramic core— using a simple hands-on method.

If the core has broken apart, it'll often rattle around inside the converter shell and produce a noticeable sound.

IMPORTANT: This test must be done with a cold engine and a cold catalytic converter. If the engine has been running, allow it to cool down completely —the converter can stay hot for a long time after shutdown. Be careful to avoid burns.

SAFETY WARNING: To access the catalytic converter, you'll need to raise your minivan. Always support it with jack stands on a solid, level surface. Never work underneath a vehicle supported by the jack alone. Also, wear safety glasses to protect your eyes from falling dirt or debris.

Let's get going:

1. 1

   Raise the minivan using a jack and support it securely with jack stands on level ground.

2. 2

   Locate the catalytic converter under the van.

3. 3

   Tap the converter with a rubber mallet or the palm of your hand.

4. 4

   Listen for any rattling or metallic clunking sounds as you tap different spots along the converter body.
   
   Bang along different spots to make sure your checking the entire unit.

Time to interpret your test result:

CASE 1: You hear a rattling sound. This usually means the ceramic core inside the converter is broken.

The catalytic converter is damaged and likely restricting exhaust flow —especially when accelerating. It'll need to be replaced.

But don't stop there: if the engine has an underlying issue like burning oil, coolant entering the cylinders (blown head gasket), or running rich from a fuel system problem, you'll need to fix that too —otherwise, the new converter will fail all over again.

CASE 2: No unusual noise. So far so good —the converter's internal ceramic core is intact.

For our next test, we'll begin checking the performance of the rear O2 sensor. Go to: TEST 2: Checking The Performance Of The Rear Oxygen Sensor.

TEST 2: Checking The Performance Of The Rear Oxygen Sensor

For our second test, we're gonna use a generic scan tool with live data mode to monitor the voltage signal from the rear O2 sensor.

If the catalytic converter is working properly, this sensor will produce a slower, smoother voltage signal (compared to the front sensor), gently swinging between lean and rich.

In a nutshell:

• Lean: Sensor output below 0.5 Volts.
• Rich: Sensor output at or above 0.5 Volts.

If the converter is no longer doing its job, the rear O2 sensor will typically show a high and steady voltage —at or above 0.5 Volts— with little to no fluctuation.

If you're unfamiliar with the terms "lean", "rich", or "closed loop", the following sections break them down in more detail:

• Understanding "Rich" And "Lean" O2 Sensor Signals.
• What "Closed Loop" Means —And Why It Matters.

Don't have a scan tool? Need a scan tool? Check out my recommendation: ZM301 OBD2 Scanner Diagnostic Tool (at: amazon.com).

OK, these are the test steps:

1. 1

   Start the engine and allow it to fully warm up.

2. 2

   Connect your scan tool and enter live data mode.

3. 3

   Make sure the PCM is in closed loop mode.
   
   The Parameter ID (PID) labeled FUEL SYSTEM will show either OPEN or CLSD. It should display CLSD to confirm the PCM is operating in closed loop.
   
   Check your scan tool's user manual to locate this PID —it might be labeled differently— or to see how your specific model displays fuel system status.

4. 4

   Find the PID labeled O2S12 (rear sensor).

5. 5

   Observe the O2 sensor's voltage output.
   
   Check if it switches between rich and lean or stays steady above 0.5V (continuous rich output).

6. 6

   Let the engine idle and monitor the sensor for a minute or two.
   
   If the catalytic converter is doing its job, the rear sensor's signal will bounce between lean and rich —but mainly stay below 0.5 Volts.
   
   If the converter has stopped filtering gases properly, you'll likely see a flat, rich signal staying above 0.5 Volts.

Let's see what your test result is telling us:

CASE 1: Voltage is steady near or above 0.5V. This indicates a consistently rich condition —which is not what we want to see.

This is usually caused by one of two things:

• A failing catalytic converter.
• A faulty rear oxygen sensor.

We'll need to dig a little deeper. Head over to: TEST 3: Manually Inducing A Lean Air/Fuel Mixture.

CASE 2: Voltage slowly fluctuates from lean to rich. This suggests that both the catalytic converter and the rear O2 sensor are working properly — at least for now.

Next step: erase the P0420 code with your scan tool and take the vehicle for a road test. If the code returns (which might take a day or two), it confirms the catalytic converter is on its way out and needs to be replaced.

Also, check for any underlying issues like oil burning, coolant entering the cylinders (blown head gasket), or a rich-running condition caused by a fuel system fault. If these aren't addressed, the new converter will fail again over time.