P0420 Diagnostics -Catalytic Converter Tests (1996-2010 3.3L V6 Caravan, Grand Caravan, Voyager, Grand Voyager, Town & Country)

When a P0420: Catalytic Converter Efficiency Below Threshold code pops up on your 3.3L V6 Chrysler, Dodge, or Plymouth minivan, it means the catalytic converter is failing and no longer efficiently scrubbing the exhaust that passes through it.

In about 99% of cases, you'll need to replace the converter to make this code go away. However, it's a good idea to run a few simple tests to confirm that the converter is actually the issue.

In this tutorial, I'll walk you through how to do just that —step by step— using a basic generic scan tool with "live data" functionality. These tests can be done right in your own garage or driveway.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.3L V6 Chrysler Town & Country: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
• 3.3L V6 Chrysler Voyager: 2000, 2001, 2002, 2003.
• 3.3L V6 Dodge Caravan: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007.
• 3.3L V6 Dodge Grand Caravan: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
• 3.3L V6 Plymouth Voyager: 1996, 1997, 1998, 1999, 2000.
• 3.3L V6 Plymouth Grand Voyager: 1996, 1997, 1998, 1999, 2000.

Symptoms Of A Bad Catalytic Converter

When the catalytic converter starts to go bad on your 3.3L Chrysler, Dodge, or Plymouth minivan, the symptoms can range from barely noticeable to hard-to-ignore performance problems.

Sometimes its internal filtering element breaks apart and clogs the exhaust. Other times, it just stops doing its emissions job.

Either way, a faulty converter can hurt performance and fuel economy. Here are the most common signs of a bad catalytic converter:

• Check engine light with a stored P0420 code: The PCM monitors catalytic converter efficiency using the oxygen sensors. If efficiency drops below a set threshold, it sets the P0420 trouble code.
• When this code appears, it's a strong indicator that the catalytic converter has failed.
• Noticeable power loss, especially under load: If the converter breaks up internally, it'll partially block the exhaust flow, which causes backpressure and reduces engine performance —you'll feel it during acceleration or climbing hills.
• Drop in gas mileage: A broken converter blocking exhaust flow forces the engine to work harder to push the exhaust out, which leads to increased fuel consumption and lower MPG.
• Rattling sound from under the vehicle: A converter with a damaged ceramic core produces a distinct rattle. You'll especially hear it at startup or when you rev the engine.
• Rotten egg smell or strong sulfur odor: A failing catalytic converter often gives off a sharp, unpleasant "rotten egg" smell when the engine is under load.
• Overheating converter: In some cases, a clogged or overloaded converter can get red-hot —noticeable especially after dark as a red glow emanating from underneath your minivan.

What Causes Catalytic Converters To Go Bad?

The catalytic converter —on your 3.3L V6 Chrysler, Dodge, or Plymouth minivan— is built to be durable, but it can get damaged and lose its ability to filter exhaust gases due to a range of engine mechanical or fuel injection issues.

Here are some of the most common reasons the catalytic converter fails:

• Engine misfires: If a cylinder isn't firing properly, raw fuel can end up in the exhaust, where it overheats and damages the converter's internal structure.
• Burning oil or coolant: Worn piston rings or a blown head gasket can let oil or coolant into the combustion chamber. When these fluids burn, they coat the converter's internal surfaces and reduce its ability to clean up exhaust gases.
• In high-mileage engines, this is probably the number one cause of catalytic converter failure.
• Fuel-rich conditions from leaking injectors: When fuel injectors leak, they deliver more fuel than the engine needs. This excess ends up in the exhaust, overworking and raising the temperature of the catalytic converter until it overheats and fails.
• Fuel-rich conditions from faulty engine management sensors: A bad sensor —like a malfunctioning MAP sensor, coolant temp sensor, or O2 sensor— can cause the PCM to inject too much fuel. If the issue isn't addressed, this excess unburned fuel reaches the converter, overheating and eventually damaging it.
• Bad engine coolant thermostat: If the thermostat is stuck open, the engine runs too cool. As a result, the PCM adds more fuel —it's part of its default programming. Over time, this fuel-rich condition can overload the converter, overheat it, and eventually kill it.
• Physical damage: Because the catalytic converter is mounted underneath the minivan, it's vulnerable to road debris or impacts —like bottoming out in a pothole— which can crack or shatter the ceramic core inside.

Replacing the catalytic converter without fixing the original cause of the failure is only a temporary fix.

Eventually, the new converter will fail the same way. So if you're dealing with a P0420 code, keep this in mind —and make sure to check for any underlying issue that may have caused the problem in the first place.

TEST 1: Checking For A Broken Catalytic Converter

Because of its location underneath the minivan, one of the most common ways the catalytic converter fails is when the ceramic core inside cracks or breaks apart.

When that happens, the loose pieces will usually rattle around inside the converter housing —you'll especially hear it when you start the engine or step on the gas to accelerate.

For our first check, we're going to tap on the catalytic converter using a rubber mallet —or the palm of your hand if you don't have one— and listen for any rattling sound coming from inside the cat.

IMPORTANT: Only perform this check when the engine is completely cold. The exhaust system can stay hot for a long time after shutdown.

SAFETY WARNING: You'll need to raise your minivan to access the catalytic converter. Always use jack stands on solid, level Ground. Never crawl under a vehicle supported by the jack alone!

Here's how to do it:

1. 1

   Raise the minivan and support it with jack stands.
   
   NOTE: Make sure the engine is completely cold. Also use chocks and jack stands for safety.

2. 2

   Gently tap the body of the converter with your palm or a rubber mallet.
   
   Don't hit it with too much force —you just want to jostle the inside slightly to check for loose parts.

3. 3

   Listen closely for any rattling or clunking.
   
   Tap along different spots to make sure you've checked the entire unit.

Let's find out what your test result means:

CASE 1: You hear a metallic rattle sound coming from the catalytic converter. That means the ceramic core inside has fractured or broken apart.

Once that happens, those pieces can shift and start to block the exhaust —leading to power loss, poor performance, or even stalling.

CASE 2: No rattling noise is heard. That's a good sign the converter is still structurally sound.

To continue troubleshooting the P0420 code, head to: TEST 2: Checking Rear O2 Sensor Performance.

TEST 2: Checking The Performance Of The Rear Oxygen Sensor

The job of the rear oxygen sensor —the one mounted after the catalytic converter— is to report how well the converter is cleaning up the exhaust gases.

When everything is functioning correctly, the rear O2 sensor will show subtle voltage swings between rich and lean conditions —similar (but not as frequent) to the front O2 sensor.

But if the catalytic converter isn't doing its job anymore, this sensor will usually display a consistently rich signal —typically stuck at or well above 0.5 Volts.

In this test section, we're gonna check what the rear oxygen sensor is seeing and reporting. What we want to find out is whether it's showing a continuously rich exhaust signal coming out of the catalytic converter —a possible sign that the converter is no longer doing its job effectively.

Also, if you're new to terms like "rich", "lean", or "closed loop", don't worry —I'll break those down in the next sections so everything makes sense.

• What "Rich" And "Lean" Mean On An Oxygen Sensor.
• What "Closed Loop" Means —And Why It Matters.

NOTE: This is the first of three tests that work together to verify whether the rear oxygen sensor is doing its job —and whether its signal can be trusted to point the finger at a failing catalytic converter.

IMPORTANT: Make sure the engine is fully warmed up and running in closed loop mode before starting this test. You'll also need a scan tool that supports live data so you can monitor the sensor's voltage readings in real time.

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

Here's how to run the test:

1. 1

   Start the engine and let it warm up completely.
   
   You want the PCM to enter closed loop mode —check your scan tool to confirm that it has.

2. 2

   Plug in your scan tool and access the "live data" section.

3. 3

   Look for the data stream labeled O2S12.
   
   NOTE: This is the rear (downstream) oxygen sensor —the one located after the catalytic converter.

4. 4

   Watch the voltage readings from O2S12.
   
   Pay attention to whether it fluctuates between high and low values (rich and lean) or stays mostly steady above 0.5 Volts.

5. 5

   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.

Here's how to interpret the rear O2 sensor output:

CASE 1: The rear O2 sensor voltage holds steady near or above 0.5 Volts. This typically means the catalytic converter isn't doing its job effectively.

It usually means that unfiltered exhaust gases are passing through, and the downstream sensor is picking that up. If this pattern holds, the converter is no longer reducing emissions properly —it's either degraded or completely failed.

Or, it could mean that the rear oxygen sensor is failing and not responding accurately.

To find out, we need to head over to: TEST 3: Manually Inducing A Lean Air/Fuel Mixture.

CASE 2: The rear O2 sensor voltage fluctuates between rich and lean, just like the upstream sensor. That's what we want to see —it tells us the converter is actively working to clean the exhaust gases.

However, if a P0420 trouble code still shows up, it's possible the converter is just starting to decline. It may still perform well at idle, but struggle when the engine is under load or running at highway speeds.