I repeatedly get a diagnostic trouble code of P0420 on 1997 Ford Ranger with 2.3L and standard transmission. Catalytic converter was replaced a year ago. Muffler wasn't. The engine (160,000 miles) doesn't use any appreciable amount of oil and shows no indications of general failure. This never occurred until I moved from the Southeast to the West at an altitude of 4,500 feet. Could the engine be running too rich? Is there a high altitude component that needs to be added or should the onboard computer compensate for possible rich running at 4.500 feet? And what is meant by "Bank 1" (as opposd to "Bank 2")?DGH

 

Welcome to FTE.

Cylinder bank one, or two, refers to the row of cylinders on V engines, with bank one containing front most spark plug #1, from which the engines spark timing is checked. Bank-1 = Passenger side, Bank-2 = Drivers side, on fore/aft mounted engines/rear wheel drive vehicles, like our Rangers.
Front wheel drive vehicles, called "Sidewinders", like the Taurus, Cylinder Bank-1 = Firewall side, so #1 cylinder is located on the passenger side on the firewall bank.
Cylindr Bank-2 = Radiator side & #4 cylinder is located on the passenger side.

Your trouble code for low cat efficiency suggests that the after cat converter O2 sensor is unhappy for some reason.

Could be a cat converter problem, or maybe the after cat O2 sensor is lazy. Have any of the O2 sensors ever been replaced????

Was the cat converter you replaced a OEM Ford, or an aftermarket brand?????

Not sure about the altitude adjustment question, but it seems to me that the MAF sensor, if working propery, would be able to compensate for altitude.

Have you tried disconnecing the B- cable for 20 min or so, to wipe the computers memory, so it can begin to learn & build new fuel trim tables at the new altitude???

Some thoughts for pondering.

 

Mercedes recomend to replace all O2 sensors if cat fails. I think they are right.... Old and lazy O2 sensor cause these problems. And not only O2 after cat converter.... PCM compare O2 sensors signals before and after cat. You need a 2 chan. O-scope to compare O2 sensors waves (first oxy sensor usually has a fluctuating waveform with 1-4 sec. wave, waveform of the sensor behind the converter should be more steady). Ford VCM tool use OBD-II data actual values of O2 sensors voltages and makes a chart on displa of laptop... but VCM is not a cheap toy. I use universal OBD-II scan software with ELM327.

 

The catalytic converter was not a Ford OEM, but from a muffler shop chain in Georgia. Should I suspect it?I have not tried disconnecting the battery cable to erase the computer's memory. A Haynes manual tells me the engine will run roughly until the computer "learns" again. I did not know how roughly it would run. I will try that.I understand from what has been written that an inline four has only one bank. There is no "Bank 2" on a Ranger 4. Right?Could a deteriorating muffler cause the symptoms I am experiencing with the engine? Probably not.I wish I could eliminate the catalytic converter, but I know that is against the law and I AM a law-abiding citizen. DGH

 

Inline 4 banger = one bank of cylinders, so no bank 2 on a inline four cyl engine.

I suppose you could have an exhaust leak & that also might upset the after cat O2 sensor.

Right now I wouldn't worry about the cat converter.

Again, have you ever replaced any of the O2 sensors????

 

Ok, here is the approach I recommend you take. If you have access to a code reader or scanner that will let you look at the long term fuel trim data. There are three possible results. If it is within ±2%, the vehicle is running properly and the cat failure is caused by something other than a fuel control problem. The engine is basically running almost perfectly.

The second option is that the value is a negative number and is greater than -2%. There are only a small handful of things that can cause this. They are, leaking injectors, high fuel pressure, plugged fuel return line, ruptured fuel pressure regualtor diaphram, and a few other things. These problems are easy to track down, since they always occur in this system, and a few simple tests can reveal the problem.

The third option I list last because it is the most common, and also can have the most causes, requiring me to write more about it. This third issue is that the fuel trim is greater than +2%. This can be caused by a large number of things ranging from faulty O2 sensors, to a failed fuel pump, to a bad MAF, misfires, vacuum leaks, intake leaks, upper exhaust leaks, etc. because there are so many more causes, it is a little harder to diagnose.

However, the most common cause is the failure of the upstream sensor(s). This sensor is used to fine tune the fuel adjustment. The problem with this is that the O2 sensor has the final say, and if it goes bad, it invariably runs rich, but often won't trigger a code until it gets really bad. Because this sensor is so critical and can easily damage other components, it is recommended that this sensor be replaced every 100,000 miles, especially if the fuel trim is outside 2%.

The second thing is that the upstream sensor can be lied to in a number of ways. Upon first glance, the system seems simple enough that it would not easily be defeated. But all too often things go wrong that produce false signals in the O2 sensor, triggering the addition of more fuel. One is poor combustion. Normally the oxygen in the cylinders is burned when it reacts with the fuel. But if the combustion is not complete, it results in extra oxygen entering the exhaust where the O2 sensor detects it, and interprets it as a lean signal. this is why replacing spark plugs is important, because worn plugs may still light the mixture, but also cause incomplete combustion. Another common cause is using high octane fuel, which burns more slowly, preventing it from burning completely in the combustion chamber. Use the octane rating recommended in the owners manual for best performance. Another common cause, which is not really a cause by itself, but rather the result of other things is carbon deposits. These deposits interfere with combustion, resulting in increased oxygen levels in the exhaust. The computer responds by adding more fuel, which often results in more carbon deposits. Removing these deposits can restore a lot of performance and improve combustion. there are so many other things that can cause this type of reading, and I can't cover them all here. But hopefully you have a place to start.

For the record, reports like yours with the cats failing several months to a year after originally being replaced are familiar to me. That is why we have a better longer lasting catalytic converter, which is also often less expensive that hyped up performance brands. It amazing how much royalty fees you have to pay for performance labels and endorsements from race car drivers. Doesn't make the product better, but it sure fools about 90% of the public.

 

To answer several of the questions posed.I bought this truck in March 2007, after which I had much very expensive maintenance work done on it by a Ford dealership in Columbus, Georgia. The plugs (all 8) were replaced at that time, as was everything that indicated any problem during bench testing. I believe the engine haas had less than 10,000 additional miles since all of the work.I am not aware that either oxygen sensor was replaced during the work, though I could be wrong. The catalytic converter was replaced during that initial period of ownership--again, by a natonal muffler repair shop. The inline fuel filter was replaced too.This condition that causes the engine light to come on always occurs when I am pulling a mile-high grade. The truck, in 4th or 5th gear, begins to feel weaker and weaker and in a few minutes the orange light is on. Because altitude causes an engine to run rich, I have assumed this might be happening. That would foul the O2 sensors, I guess. But as your answers indicate, there are many possible causes for the condition I am experiencing.I know that part swapping, using parts from a discount auto parts store is the backyard mechanic's solution sometimes. I could acquire new O2 sensors and put them on, but the problem might not be resolved.I suppose a better solution would be to acquire one of the meters that reads the error codes and then try to make logical deductions about likely causes.Thanks again for your advice. Were I near the shops some of you guys run or work in, I suspect that the problem could be resolved easily.DGH

 

Considering the mileage on this puppy & your recent change in altitude, If your going to try pulling the B- battery cable to wipe the computers fuel trim tables, why not also pull & clean the MAF sensor with a non residual cleaner. CRC makes one formulated for cleaning MAF sensors.

If the air filter is due, change that too, all before hooking the B- cable back up, so the MAF sensor has a unrestricted flow of air & the computer is getting good PID's from the cleaned MAF sensor.

If you can come by, or loan a scan tool that'll monitor PID's, like a Actron CP9145, ect, you could make a judgement as to the O2's switching speed & if they are they lazy & need replacing.
With the mileage on them, they belong on your suspect list as BearRiver has said.

If your going to replace the O2 sensors, also do that before rehooking B- battery cable, so the computer is rebuilding fuel the trim tables at your new altitude, with good input.

Nice trouble shooting tutorial Bear River!!!!

More thoughts for consideration.
Let us know how it goes.

 

I left battery cable off last night and put it back today. The engine seems to be running better. Perhaps the computer has compensated for the thinner air here. In any case, I will pursue your others suggestons. I wonder how much I should expect to pay for the oxygen sensors at my friendly discount auto parts store? I do not have a Ford dealership closer than 70 miles.DGH

 

Good to hear you believe a battery disconnect/memory wipe has helped.

Check one of our site sponsors, Direct Ford Parts, a link to them is in the above left margin. I believe a Ford O2 sensor runs about $55 & change from them.

 

Argh, why do you insist on not pulling up the long term data. Without that value, you really don't know what the computer was doing. By disconnecting the battery, you have cleared that value, so now you are acting blind until the problem returns. The long term fuel trim value is the amount the computer was adjusting the injector pulse width versus what it should be. Catalytic converters and the diagnosis of catalyst related problems is my specialty.

I am trying to help you figure out what is wrong in the simplest way possible. The simplest method is with the fuel trim data. This value gets you pointed in the right direction. Unfortunately this does require a code reader that can retrieve such data, but trying to figure out what the underlying problem is is like a doctor trying to find out if a patient has an underlying heart condition without using either an EKG or a stethoscope.

As far as altitude effecting its operation, not a chance. The MAF sensor detects the volume of air entering the intake, and the computer makes the necessary calculations to put in the precise amount of fuel required. Altitude has no effect on this value. Thinner air means less air volume. This means the computer automatically adds less fuel. If the MAF is dirty or fouled, it can give incorrect readings, but altitude still has no effect on this. Altitude can effect other things, but those adjustments are also automatic.

 

the cat was changed at 165,000 with aftermarket cat. Could it be bad again. Runs fine, never rough. Any suggestions

 

Welcome to FTE.

You have resurrected a 10 year old thread.

With the mileage on your vehicle, Lots of unknowns, so the Forum would likely need Way more info to hazard a guess about what your problem may be, or what's causing it.
I'd suggest going to this forums thread index page, by toggling up to find & click on the small blue type "Ranger & B-Series" under the black header band, its atop the upper center/left side of this page.
When the forum index page opens look on the upper left side of the window to find & click on the golden + New Thread tab, to open a window for you to begin a New thread, so you get answers just for your problem.
Include the info you've posted here, along with where the vehicle is on All past & present due scheduled maintenance replacement items. like filters, plugs, wires, O2 sensors, engine or exhaust work, etc. Also post All trouble code Numbers, as they can help focus a trouble shoot.
Did the problem come about slowly over time, or suddenly after some event?
Don't answer this post here as it doesn't have anything to do with the original posters woes, answer in your new thread & we'll all see you there.