My check engine light comes on

Fault codes never say to replace parts. They only indicate the circuit or system that needs further diagnosis. People who sell parts want to sell parts. No one can diagnose a bad part without performing some diagnostic tests.

You misunderstood Doc, we're not here to degrade people-If I were you best listen-The code-is it P0420?

Hold on. You are not reading what I typed. That was the standard reply I use every day when I run into the same comment that "I got a code and I replaced the part". NO ONE else has ever taken offense at that tiny bit of information because it is not common knowledge. Most people assume fixing cars is just that easy and they get angry with mechanics who charge for diagnostic time since "the car tells you what's wrong". It doesn't. You don't get to interpret my comments as you wish to.

Now, if you want to try to figure out the cause of the problem, we can continue, but you didn't even bother to ask a question. You got step one done already. You checked for codes. Dandy, but based on that code, a parts salesman told you to replace a part. That was the first mistake. You got another code, possibly the same one but you didn't say. At that point it should have been obvious the salesman was guessing and as long as your wallet keeps on spewing dollars, he's going to keep on making suggestions. Second mistake.

There are over two dozen fault codes related to the oxygen sensors and they all mean different things and require different diagnostic tests. You didn't list the code number so we can research it further to see what can set that code. Third mistake. You didn't even bother to list the engine size but we would have gotten to that in the course of the conversation. I will never understand why so many people keep those details secret, then get angry when they don't get the expected response. Fourth oversight. You obviously do not know how the oxygen sensor system works but that is to be expected. This forum is mainly for do-it-yourselfers who know little about cars. THAT'S who we're here to help. Had you come here earlier we would have prevented you from buying a catalytic converter unless testing showed it to be ineffective. That's not a common problem.

Your observation that you are not a mechanic and you "do the best you can" will get you into trouble on most of these vehicles. Every day I read about people causing damage to wheel bearings, computers, transmissions, and every other part of their cars. They aren't the same as they were years ago. I applaud anyone who wants to learn more about their car and try to diagnose it them self, but the first step is understanding how the systems work. I can help with that. You'll be better prepared to do as much as possible yourself without fancy diagnostic scanners once you're armed with that information.

First of all, which engine do you have? You said you replaced the upstream and downstream O2 sensors. That implies you have a four cylinder engine. If you have a V-6 or V-8, you have four O2 sensors. Which side of the engine did you replace those sensors on? The front and rear ones do entirely different things and will set different codes. Do you remember the exact code number? Was it the same number both times?

Another point of confusion; one that gets mechanics frustrated and customers angry with mechanics is how some fault codes set. There is always a long list of conditions that must be met to set a code. In almost all cases, one of those conditions is that certain specific other codes can not already be set. That's because the Engine Computer constantly compares sensor readings and reconciles them with each other. When a code is set related to something the computer compares to a second parameter, it can't test that second parameter if there's nothing valid to compare it to so it won't set that second code, ... UNTIL the first problem is fixed. That's when the second code immediately pops up. That happens all the time on some cars with their anti-lock brake systems.

Once you do have the code, it will tell you whether the O2 sensor heater is grounded or shorted to voltage, whether the signal is staying lean or rich too long, whether it's not switching properly, or if it's producing no signal. Not switching properly and staying in one state too long have nothing to do with a bad sensor. Those conditions actually prove it's working. It is simply reporting an improper condition. You have to diagnose the cause of the condition, not replace the sensor. Many people find it easier to understand the coolant temperature sensor. If it reports the engine is overheating, you don't solve the overheating by replacing the sensor.

The next thing that is not terribly common but can drive experienced mechanics crazy is a leak in the exhaust system ahead of the catalytic converter. That is something to look at since you replaced the converter, even if you can't hear it. Between the pulses of exhaust flow, the momentum creates little pules of vacuum that can draw in outside air. The oxygen in that air will be detected by the front O2 sensor which will report a lean condition. The computer will respond by commanding more fuel to all cylinders on that side of the engine. No matter how much more fuel it commands, there will still be that unburned oxygen showing up and it will finally set a code related to the computer losing control over the mixture.

The same thing can happen from a single cylinder misfire. Unburned fuel and air go into the exhaust where the unburned oxygen is detected as a lean condition on that side of the engine.

Your fifth mistake was assuming I'm lecturing you on being dumb. As I pointed out, we work here mostly with people who know little or nothing about cars. I can list dozens of common things I know nothing about. That doesn't make me dumb. The sixth red flag is "I need to know what to try next". The last thing anyone here will do is tell you to "try this" or "try that". That is the most expensive and least effective way to diagnose a problem. If you're going to resort to that kind of shotgun approach, take the truck to a mechanic because it will get fixed faster and cheaper. That is not meant to be as sarcastic as it may sound. The exception is when you're trying to learn about your car or truck. For the average owner, seeing a mechanic is the least expensive route.

One more thing to look at is where the front O2 sensors' wiring runs down behind the engine. On more than one occasion people have found that harness to have a broken attaching clip and the wires fall onto the hot exhaust pipe and melt. The code(s) will depend on which wire melts through and grounds out first.

Happy to hear you understand my intent. Thank you. The speed sensor in the differential is for anti-lock brakes and will set a code in the ABS Computer. Code 720 refers to the speed sensor on the left side of the transmission, but when he has a problem you should notice something with the speedometer too. Since the Body Computer is also involved, it could be covering up momentary dropouts in the sensor's signal. When that type of sensor fails, it typically stays in the failed condition for long periods of time. Two-wire speed sensors have a coil of wire wrapped around a magnet. Magnets have been known to crack leading to erratic signals during vibrations and road bumps, but more commonly one end of the wire breaks off its terminal. That broken connection can repair itself when the sensor cools down.

For a very short dropout, you're more likely to find a poor connection with a terminal in a connector due to corrosion or it being stretched. Also look at the end of the sensor once it's removed to see if there's black "fuzz" or other metal particles on it. Those will reduce the signal strength to the point where one computer may still read it, (the Body Computer and instrument cluster, for example), but another computer might not, (the Engine Computer, for example, which is the one with the code).

Three-wire sensors still use a magnet and coil of wire but they also have electronic circuitry that often becomes heat-sensitive. They may stop working when hot, then work again when they cool down. Speed sensors typically don't cause much trouble. The same things affect them as do the two-wire sensors. Look first for metal chips on the end and connector pin problems. Three-wire sensors are used for crankshaft position sensors and camshaft position sensors too. Those are the ones that fail much more often.

Most code readers used by the auto parts stores that read codes for you for free are relatively basic and only access the Engine Computer. Full scanners will access all the other computers and look there for their own fault codes. Those will usually not turn on the Check Engine light. Any problem that is detected that could adversely affect emissions must turn the Check Engine light on. When that applies to some other computer, that computer will tell the Engine Computer to turn the light on. The Transmission Computer is a good example of that. While it won't affect emissions directly, if it's not shifting properly or at the right times, fuel usage could go up leading to increased emissions. With a speed sensor problem, the Engine Computer may not know when to allow the transmission to up-shift to overdrive. With the electronically-controlled transmissions in cars, no shifting may occur at all and they will default to second gear. That will REALLY increase fuel consumption.

The best approach for this type of problem is to try to get your hands on a scanner that will display live data during a test drive. I have the Chrysler DRB3 that I use on my newer personal cars but it's expensive. I just did a search two days ago for someone else and was surprised to see the price for used ones has come down quite a bit. There's lots of good aftermarket scanners out there too that will work fine for a '98 model but they never do as much as the original manufacturer's stuff will do. In that respect, the newer the scanner, the better.

During the test drive you can watch the speed sensor signal the computers are seeing. Dropouts are almost impossible to catch if they don't last long enough because those scanner readings only update about twice per second. The computers DO catch those dropouts. Where you will see them is in the "record" function. Once set up, you press the "record" button when the problem occurs. In this case you would have to press it the instant the Check Engine light turned on, and that can be hard to do. The scanner will record about five seconds of sensor data that you can play back slowly later while watching for glitches or sudden changes. Since that data travels through the scanner's memory, the recording actually starts a couple of seconds before you pressed the button.

I wouldn't spend money needlessly on sensors just based on principle if there's no related codes. They don't really cause that much intermittent trouble that they need to be replaced as a maintenance item.