Fun with PO420

[Mtrevathan85]

I'm new to this site so I hope I am posting in the correct spot. I wanted to share my experience this week with a PO420 (catalytic converter bank 1, something like that) code that I have been trying to figure out for some time. I want to spare you a lengthy explanation so I will just list the steps I took. The PO420 code is a tricky (removed) and I am by no means a mechanic, but I have youtube and I love taking stuff apart.

At first glance it seems that PO420 would be something to do with the CAT itself, but after researching I found that a faulty CAT is rare, even in older vehicles (My vibe is at 126k). I also read that this code can be triggered by sensors in the engine before the CAT even receives the fuel mixture/exhaust. It had been some time since I had serviced the vehicle so I went ahead and changed the spark plugs (much overdo) and in doing so found that coil #1 was not functioning, so I replaced coil #1 as well. So now my vibe is really humming, she's not my main ride so I haven't paid her enough attention lately. So i reset the light via disconnecting the battery. Now, on my way to school and about 30 miles into my commute the friggin' light clicks back on. So now I'm thinking its definitely the CAT, because what else, right? I had recently replaced the forward 02 sensor near the manifold so I knew that wasn't the problem. I figure either the downstream 02 sensor or the CAT. Looking at the engine, my air filter was filthy, replaced it. I'm still not confident, so I call my uncle (mechanic) and ask him. He tells me to replace the Mass Airflow Sensor because a dirty filter can damage the original over time. Presently, I am at about 30 miles in and no light , since I reset it after replacing the MAF-S. I am keeping my fingers crossed because I need this one to pass inspection. I hope that my experience can help someone else dealing with this code. I will update this post if anything changes. Please let me know if you've ever encountered this problem.

[joatmon]

Welcome to genvibe.

So far I have dealt with P0420 on 3 of these cars.

My first vibe, an 03 base, started throwing the code at about 100K miles. Tried replacing the downstream O2 sensor but that had no effect. Replaced the cat, and that fixed the problem. For about another 110K miles, until it came back. I then used a pair of spark plug anti-fouling tips on the downstream O2 sensor, which worked until I replaced the car about 80K miles later.

I replaced it with another 03 base Vibe with 157K miles on it. About 30K miles later, the dreaded P0420 code, and countered the code that time with the anti-fouling rips on the O2 sensor.

At some point my son's 03 Matrix XR started with P0420, and I did the tips again.

As I understand it, P0420 is set when the downstream sensor reading isn't lower than the upstream sensor. A non working cat doesn't affect the exhaust enough for the sensors to detect a change. I think the downstream sensor is the key player, it's only purpose is to monitor the cat. The upstream sensor primary role is to monitor lean/rich, to help the computer dynamically adjust fuel/air mix, and only secondary to set the baseline to compare against the downstream sensor. A bad upstream O2 sensor would more likely affect engine performance and efficiency than it would P0420

The spark plug anti-fouling tips work by reducing the exhaust flow around the second sensor, lowering it's readings. It's not a fix, more of a way to ignore the problem and avoid the CEL. It's also something I can get away with because I don't have periodic emissions testing here, but ecologically uncool.

Not directly related, but I bought a used Tacoma, before buying, my code reader said "not ready" so I made a condition of the sale that they would fix and hidden CELs. Sure enough, 100 miles later, P0420. First they replaced both 02 sensors, which didn't help, so they replaced the cat, and the truck has been P0420 free for 4 years/30K miles so far. (I drive the Vibe more than the truck)

For what it's worth, viewtopic.php?f=17&t=44508 mentions the best price I have seen here for a bolt on replacement cat.

[Mtrevathan85]

I fear you are correct. About 50 miles in I was thinking I was home free, but the damn light clicked on when I got to school again. I am going to leave here today and raise the Vibe and take a look at the CAT again and see if I can spot any damage. I am pretty sure it is the CAT, but I would hate to go buy a new one to find out something is wrong with the downstream 02 sensor. I am going to hook it up today and see if I cant diagnose it by looking at the wave pattern coming off the sensors. I understand what you're saying about the 02 sensor not really affecting the PO420, so I am most likely going to be pricing new parts today. Thank you for sharing your experience, this has been frustrating to say the least. I will update the post and let you know what happened :-X

[ehoff121]

Welcome to genvibe!

Just to throw in another in progress P0420 story, I've been experiencing an intermittent CEL for this code since last fall. This is after replacing the entire exhaust from the manifold back last spring, including both O2 sensors. I used the Denso O2 sensors as replacements, but they only carry a 12 month warranty and, of course, mine are past the purchase anniversary date. Also, the replacement cat had different specs than OEM.

The ECM is supposed to throw a P0420 when the downstream O2 sensor readings are out of range in three separate tests of the system, when the RPMs are between 2,000 and 3,000 for three minutes. In practice, this means you are either going up a hill at 30 mph or cruising on the highway for the tests to happen. I think three tests means three separate trips (ignition starts), but not 100% sure. If you don't meet the test conditions, the code won't set.

There is a potential that a leaky pipe or connection could allow air in the system and cause the code. I've noticed that on cool, wet days, the code is more likely to set. I can go for a couple of weeks without a code, but on a rainy day, bing! There might be something about moist air that messes with the sensor- have you seen this, too?

There was some post on another site where I read that arc welding on a car can do some random things to sensors and, coincidentally, I had a mechanic try to help me remove a broken bolt from the rear differential and he attempted to arc weld a nut on to the bolt.

The next thing I plan to check is the possibility that the valve cover gasket (being 13 years and 115,000 miles old) is not keeping oil out of the cylinders well enough and is causing the O2 sensor to foul or, at the very least, read out of range.

Good luck and please let us know if and when you figure this out.

[Derf]

Also noticed the PO420 gets set more in the cold weather. Since I have had my Vibe the P0420 has come on a few times. Almost always on cold days. When I Sea foamed the Vibe to clear the gunk out of the intake, the crud hit my Catalytic converter and decreased efficiency, noticed I got P0420 after cleaning the intake. In hind site the PCV valve was the source of most of the oil, replacing the PCV valve and checking for other oil issues goes a long way to prolonging CAT life