I have a 1997 Mazda B2300 with 175,000 miles that has an intermittent check engine light. When I check the code it is always P0420. If I leave it alone long enough the light usually goes out. The truck had the catalytic converter replaced about 5 years and 60,000 miles ago. I also had the muffler replaced in October, but I was getting this code prior to the muffler replacement. I recently bought a scanner that can monitor the fuel trim data and noticed that the long term fuel trim is generally between +16 and 18%. I replaced both oxygen sensors last week and cleaned the MAF sensor, but today I got the CEL again. The LTFT is also still the same as it was before I replaced the O2 sensors. My truck doesn't use any noticeable amount of coolant or oil and I'm not really sure where to start.

 

This link lists some causes of your code:

P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)

 

Did you change the 02 sensor located downstream (after) the cat converter? The wiring and harness connector for that particular 02 sensor should also be checked.

From the '97 Ford Manual:

 

Welcome to FTE.

Good idea to have scanned the computer for trouble codes.
The long term fuel trim info you've posted makes me wonder if you have other trouble codes stored, if so, post All code Numbers you've seen, as they can offer up good trouble shooting clues.

The P0420 is a cat converter efficiency code, that says the after cat converter O2 sensor isnt happy with what its measuring coming out of the converter & thinks the converter isn't doing its job. It might be right, as over long term with the cat converter having to process that rich a fuel mixture, it may have destroyed itself again!!!!

So you need to look upstream at whats going on with the engine, or its computer sensors, as you have, to find out what would likely cause the computer to add that much fuel long term.

With the long term fuel trim being that high, it says that the computer is adding fuel because it thinks the fuel trim is lean & lean means it thinks the air/fuel ratio has too much air for some reason & is adding fuel to balance things out. So, lets think some more about what could cause the computer to believe that, or that to actually be happening.

As you guessed, with the mileage on the vehicle it may have been because the upstream before cat O2 sensor switching speed was old & slow, or the MAF sensor may have been dirty & with the mileage on the vehicle, those were reasonable guesses, but don't seem to have been the problem, as you report that nothing has changed since replacing Both O2 sensors & cleaning the MAF sensor.

SO, thinking about this some more, maybe the computer thinks the air/fuel mixture is too lean because of an upstream vacuum leak, like an intake manafold leak, or maybe a lean shot of fuel from dirty fuel injectors, or low fuel pressure, or low volume/delivery rate because of a dirty fuel filter, pinched fuel line, faulty fuel pressure regulator, weak fuel pump, clogged up in tank pump pick up screen, low operating voltage to the fuel pump, are some things that come to mind???? EDIT: WOOPS, forgot to have the IAT & ECT sensors put on the suspect list, as if either are putting out a corrupt PID to the computer, they'll lie about correct air or engine coolant temp & that'll have the computer mess up the fuel injectors squirt time & that'll mess with fuel trim, so add those two items to your suspect list until you can confirm their PID output is ok with your scantool.

So, with your scantool, post up the short term fuel trim, O2 sensor voltage switching range & speed & any other trouble code Numbers you've seen.

With your fuel pressure gauge connected at the under hood fuel rail schrader valve test port, measure fuel pressure & volume delivery over time. If low, when last was the fuel filter replaced???? If the fuel filter is likely ok, measure under load voltage drop to the fuel pump at the in cabin inertia switch.

Check out the PCV vlave & its connecting rubber elbow hose, as it likes to dryrot & leak vacuum. With this many miles, the PCV valve is Way past due for replacement. Being down stream of & not monitored by the MAF sensor, this system if leaking air, can corrupt the air/fuel ratio & have the O2 sensor report a lean mixture & confuse the computer about what to do about fuel trim.

Since you've had the exhaust system worked on, it belongs on the suspect list too, as any leaks ahead of either O2 sensor can corrupt their PID output to the computer.

Whatever you do, Don't replace the cat converter again, until you get the fuel trim problem fixed.

A bunch more thoughts for consideration, let us know how your trouble shoot goes.

 

I replaced both the upstream and downstream O2 sensors.

I don't have any other codes stored and can't remember having ever seen any codes other than this one.

I logged some data using my scan tool on my way home from work today. It's at this link:
https://www.dropbox.com/s/9at1gpbjm6...1_17-46-42.csv
I'm pretty new to this so I don't really know how what to look for in this data.

The PCV hose looks fine. I think I'm going to go ahead and replace the hose and the PCV valve, since it's supposed to be replaced every 100,000 miles.

I don't have a fuel pressure gauge. I'll try to pick one up soon.

 

I did a couple of things today. First I checked the vacuum, which was fine. I did notice that the hose between the EVAP valve and the carbon canister is missing, but is this a big problem or at all related to my fuel trim and P0420 code issues?

The thing that prompted me to get exhaust work done in October was not the leaking muffler, but actually a leak between the exhaust manifold and the catalytic converter. The shop said they just had to crank on the bolts between the cat and the exhaust manifold. I thought the problem was fixed, but I have been hearing a slight buzzing sound lately, so I checked for leaks today.

I got under the truck while it was idling to see if I could hear/feel any exhaust leaks. It sounds to me like there might be a leak in the catalytic converter itself. I will do some more investigation later this weekend when I have someone to help me. I don't think this would cause the fuel trim issue. From what I understand the upstream O2 sensor is responsible for fuel trim, so a leak in the cat shouldn't affect fuel trim. Is this right?

 

You need to get the missing vapor recovery line put right.

 

I used to work in the catalytic converter industry.

When you get a P0420 code it pretty much means the cat is toast and needs to be replaced, but usually the damage tot he catalyst was caused by a problem, that may or may not set a code. The upstream sensor is a common culprit, which you say you've already replaced. Was it replaced before or after the converter code came on? When I used to sell them, if the sensors were not replaced prior to or at the same time the cat was replaced, we could not honor any warranty claims. Replacing the sensor after the light comes on is useless because the damage is already done. Replacing the downstream sensor rarely has any effect at all. I ran into many cases where the downstream sensor was replaced or checked, but at the end of the day, catalytic converters usually fail because they are being forced to operate outside their design parameters, i.e. the mixture is either too rich or too lean. The upstream sensor controls the fuel trim, so if it getting too rich or too lean, the downstream sensor has nothing to do with that.

But obviously, the O2 sensors are not the only factors that come into play and affect the operation. Vacuum leaks cause issues, including leaks in the evap system, as do excessive spark plug gaps, misfires, burned exhaust valves and seats, excessive oil consumption, antifreeze contamination etc. Some of these issues may not trigger codes, so it takes a bit of good old fashioned detective work.

I will also add that many brands of aftermarket converters are junk, and you are lucky to get more than a year or two out of them. Some are better than others, the aftermarket catalytic converter industry is not a commodity based industry. In commodities, all products are roughly equal, and the lowest price is usually the best deal. In aftermarket converters, the quality differences between brands is huge. The coating technologies used, the product line, the types of materials used, etc. make a huge difference in the longevity. The lowest price is usually not a good deal, but the highest price is not necessarily better either. Some very expensive brands have you paying for the name, and yet they are some of the worst garbage out there.

If your fuel trim indicates a lean mixture, then any vacuum leak is a high probability culprit. Fix any and all vacuum leaks. Fixing the leaks is not likely to make the P0420 code go away, but you can verify the fix by see if the fuel trims go back to where they should, close to around 0%. As a general rule, if we ever saw a cat we sold fail, if the fuel trims not he car were consistently beyond 5% the claim was denied, the warranty was always conditional upon he car being properly tuned, and properly tuned cars have LTFT% fairly close to zero. Physical examination of the failed product would ALWAYS confirm our reasons, the cars were running extremely rich and coated in heavy carbon deposits. When your system indicates lean, it can actually mean rich. If the computer thinks you are running lean, it tries to add more fuel. Too much fuel can result in a rich mixture.

 

It definitely sounds like I have two separate issues. The first is that the fuel trim is very high. The second is the catalytic converter code which sounds like it might be caused by the first issue.

I only replaced the upstream oxygen sensor about a week ago but I have been experiencing this code for about a couple of years.

The replacement catalytic converter was purchased on eBay about 6 years ago, so I'm definitely not confident that it was a high quality part.

I think I am going to go ahead and replace all the vacuum hoses and the PCV valve. When I measured vacuum the other day I was getting about 20 inches Hg, but I did notice that the vacuum measurement was moving around a little at idle.

The truck also doesn't have a very smooth idle. The tach moves around some at idle, sometimes as much as a couple hundred RPM. Also, when coming to a stop when I depress the clutch the RPMs occasionally drop to about 500 and the engine sounds like it is about to stall. Is this an indication of a vacuum leak?

Thanks.