by Sam Dillinger (comments: 2)

2005 Chrysler Pacifica P0420


The next submission we’re going to be looking at is from a customer who has a 2005 Chrysler Pacifica with a 3.5 liter V6 engine and is getting the Pcode P0420. SPOILER ALERT: the culprit was a bad catalytic converter. Why am I saying that so early in our look into the issue? Easy, this is by a very wide margin the most common cause of failure for this Pcode. There are certainly other issues that can cause it, but they’re definitely the exception and not the rule, for this vehicle and engine combination. However, read on, and I’ll explain why a proper diagnosis is so important even for Pcodes like this, that have a specific failure almost all of the time.

Let’s begin by looking at what this Pcode is telling us. P0420 means “Catalytic converter 1\1 efficiency failure”. As you can see, the catalytic converter is named directly in the Pcode. That’s not what makes it the most common failure, and it’s important to understand what the Pcode is actually telling you. A lot of people see the words “catalytic converter” and their diagnosis stops there. This can be a really expensive way to find out that there are other causes. So what is it actually saying? It’s telling you that the catalyst is no longer converting the exhaust gasses entering into it, into the non-harmful gasses that exit it. It’s monitoring the amount of Oxides of Nitrogen and Hydrocarbons that enter the unit versus what is leaving. If your downstream oxygen sensor is not reading right, the catalytic converter could be operating correctly and due to the faulty reading from the sensor, the PCM, Powertrain Control Module, can interpret that information as a failed catalyst. So the most important first step is to know that this Pcode doesn't automatically mean a bad catalytic converter.

Now we’ll look a little further into what other causes there are for this condition. The list is actually not very long: an engine mechanical malfunction, fuel system malfunction, an exhaust leak, or a failed oxygen sensor.

First is the engine mechanical. If you’ve got a vacuum leak, a burnt valve, a broken/cracked piston ring, an engine out of time, or similar condition, this will throw off the fuel system by not burning the fuel as completely as it should be. This can happen in one or multiple cylinders. The important part about any of these failures, as it pertains to our discussion of this Pcode, is that all of these will case noticeable symptoms, drivability problems, and most likely other Pcodes. In this case, the vehicle ran fine, but every time the check engine light was reset, it would come back in a day or so.

Next is a fuel system malfunction. This can include low fuel pressure, a sticking or dirty fuel injector, an electrical problem such as a failed sensor, or a vacuum leak. As with the engine mechanical failure, all of these will normally have very noticeable symptoms, along with other Pcodes being set. I will make a special note of a “dirty” fuel injector. The opening on the nozzle of a fuel injector where the fuel actually sprays out is incredibly small. It’s hard to accurately describe just how small it is, but to try and give some visual aid, if a single grain of sand was somehow able to make it past the filter and pintle built into the injector it would render the injector completely useless, stopping the flow of fuel from that injector completely. So when someone tells you that you have dirty fuel injectors, it’s not the same as a dirty air filter, or clogged screen in the traditional sense. That being said, they can get a small (really small) buildup of sediment at the filter of the injector or at the opening. A pressurized fuel cleaner will normally take care of the problem. I’m mentioning this one in particular because it doesn’t always cause a misfire when this happens and can cause the P0420 to appear. This is one of the main reasons why a proper diagnosis is so important, even in the case where the cause of the Pcode is a particular component 95% of the time, as is the case with this particular Pcode.

Next is an exhaust leak. I’m not going to spend a whole lot of time explaining this issue, because if you've ever had a leak in your exhaust system, you know that it’s pretty evident. It gets loud pretty quickly and is hard to ignore, especially as it gets bigger. The only thing to note here in the context of our conversation is that an exhaust leak before the catalytic converter will throw off the readings of both oxygen sensors and that is specifically what can cause this Pcode to appear. As I said though, an exhaust leak is pretty easy to detect just by listening to your car as it runs.

Last is a failed oxygen sensor. These are more commonly known as O2 sensors. A lot of people get tripped up on the possibility of a failed O2 sensor because they don’t understand what it’s purpose is. I’m speaking from a purist stand point on this issue. The O2 sensor accomplishes 1 thing only: it measures the oxygen content of the exhaust that is flowing past it. That’s its sole function. This is why it gets confusing- the PCM, Powertrain Control Module, reads the information from the O2 sensor and makes adjustments to several different programs that it runs based on this information. For example: if the upstream O2 sensor reports that there is too much oxygen in the gasses it’s sampling, the PCM will allow more fuel to be injected into the engine as it reads too much oxygen = not enough fuel. If there was an exhaust leak before the gasses reach the sensor, it would trick the sensor because there would be more oxygen present due to a lack of post-combustion gasses. So a failed O2 sensor can hide itself as a fuel problem in this instance, as well as a decent amount of other issues.

There is also some confusion about the designation of the O2 sensors. It’s written like this 1/1, 1/2, 2/1, and 2/2. This is most common although there can be 3/1 and 3/2, this is extremely rare and only for some diesel engines. So what does this mean? The number on the left of the slash mark is telling you which side, or bank, of the engine you’re dealing with, while the number on the right of the slash denotes upstream or downstream. Conventional wisdom has stated for years that the 1 or 2 on the left of the slash means left or right bank. This is actually not true and it’s very important to get this right so that you're dealing with the correct sensor. It actually means bank 1 or 2. Bank 1 is whichever side of the engine that has cylinder 1, and of course bank 2 is the side of the engine where cylinder 2 is located at. This is important to note because cylinder 1 may or may not be located on the left side of the engine! So to put it all together- 1/2 would mean bank 1 downstream O2 sensor. The upstream and downstream are referring to the catalytic converter. The upstream O2 sensor is before the catalyst (some are mounted directly into the catalytic converter but still count as the upstream sensor) and the downstream O2 sensor is post-catalyst. One final note about this: a 4 cylinder engine only has 1 bank so it only has 1/1 and a 1/2 sensors. The rest are on “V” engines such as V6 or V8. Whew, that’s a lot of information!

So now to come full circle and look at the original condition, we’ve got a V6 engine with a 1/1 catalytic converter efficiency failure, P0420. Without the rest of the information the customer gave, this is not enough to make an educated guess on what the failed component is. When I spoke to this person more, I found out that the O2 sensors had already been replaced and the condition returned. The car was running perfectly normal with no other observable symptoms. The check engine light came back within a day or so every time it was erased, and there wasn’t any smoke coming from the tail pipe, and no other Pcodes showing up. Taken with the fact that the catalytic converter is the most common failure, that’s where we ended up. After the customer replaced the converter, the light did not come back.

Believe it or not, the information I gave in this article is actually incomplete and almost as basic as I could make it, while still covering the most important parts! So with that, if my members have any further questions, please drop me a line at and I’ll be happy to give some more in-depth information, as well as answer specific questions!

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Comment by Bill White |

Very good information and diagnoses fit my Pacifica to a “T” .

Reply by Sam Dillinger

Thank you, I’m glad we could help! 

Comment by Malik |

Great article, this describes the p420 code in good detail and I appreciate the explanation. Thanks!

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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