Fault codes must be looked at as a tool to help make a diagnosis, not as the diagnosis itself. I know we're in trouble when someone asks for the text definition for the fault code they got but they don't describe the symptom or problem. Those are the people who think the codes tell the mechanic which part to replace. While that does end up happening about half of the time, the code only describes what improper operating condition the computer is seeing or which circuit needs further diagnosis.
I built a lot of "bugs" into cars for my students to troubleshoot. In one car, there were 8 different problems that would cause one of two different throttle position sensor codes to set. Only two of those were for a defective sensor itself. The other six set the same two codes but were caused by wiring or connector pin problems. The typical do-it-yourselfer would just replace the sensor, (and he would get lucky about half of the time, but if it didn't solve the problem, he was lost unless he knew basic electrical theory, AND he knew how that circuit worked, AND he understood what had to happen to set that code. I never allowed my students to use the troubleshooting charts in the service manuals. That lets a book written by a smart person to lead them to the cause of the problem. Instead, once they understood electrical theory, it only took three or four minutes to learn how each circuit worked and what it took to set the code. Now they were the smart person, and they could transfer that knowledge to any brand and model of car and be ready to diagnose any of them without having to search for a manual.
Learning all that theory is not practical for a do-it-yourselfer. That's where service manual flow charts and troubleshooting trees come in handy. That's where this web site is valuable.
Now, let me go in a different direction for a minute and add something about those codes. They are just one of many tools the mechanic has at his disposal but they don't always tell the whole story. As an example, most sensors run on 5.0 volts. The acceptable signal voltage coming out is anything between 0.5 volts and 4.5 volts. Anything under 0.5 volts or over 4.5 volts will trigger a fault code. It's by those voltages that the computer knows something is wrong. The throttle position sensor has mechanical stops that prevent it from going outside the acceptable range at idle or wide-open-throttle, but if the moveable contact encounters some dirt or anything else that causes an intermittent connection, there is a method used inside the computer to force the signal voltage to go to a known defective voltage to force the computer to set a code. That intermittent connection can cause a hesitation, a stumble, or sputtering at the speed where that connection is lost, etc, but without that code, how would you ever figure that out when it doesn't act up while you're standing alongside the car? WITH that code, we at least know the problem is related to that sensor and its wiring so we can operate it in hopes of seeing the problem occur. Seeing is believing in my world. Too many people, mechanics included, will just change the sensor and hope that takes care of the problem. I would probably do the same thing with my own vehicle but there's a big difference. First of all, I drive an '88 minivan because I love its lack of computers and it's been uncommonly reliable. As such, I have spare parts like sensors on hand that I've accumulated over the years. Popping a new one in to see if it fixes the problem is faster because I'm the one driving it every day, I already have the part so I don't have to buy it and hope I can return it later, and if the problem still continues to act up, I know I've eliminated the easy thing; now it's on to diagnosing the harder things. That is not a practical way for a mechanic to approach your vehicle. Even if they said they would install a new part, let you drive it a few days, then refund the cost and try the next part for a few days, most people would not go for that approach. We have more people angry because they have to make multiple return trips than people who are angry when we install potentially more parts than are needed to insure the problem is fixed. Given the choice verbally, most customers opt for "more parts or services" vs. "This might not be the entire cure". I'm just the opposite. I can pinch a penny until it smokes, but there are times when doing more than the bare minimum is in the customer's best interest in the long term.
Getting back to those sensors, all domestic Chryslers use a MAP sensor as the main sensor to calculate the fuel needs of the engine. All other car brands use a more complicated and more troublesome mass air flow sensor, but they often use that MAP sensor as a backup. Either of those sensors can start to fail by sending the wrong signal voltage but as long as that voltage is within 0.5 and 4.5 volts, no fault code will be set. This is where the mechanic has to understand how they can cause severe running problems without setting a code so he knows where to start looking. There will often be some other code such as "running too lean" or too rich, but those don't even point to a circuit, just to a condition. The mechanic needs knowledge and experience to solve those, and he has to often "try" a new part to see if it is the solution. In the case of the MAP sensor, a voltage that's off by 0.2 volts will appear to be normal when live data is viewed on a scanner, but that's enough to cause a severely lean or rich condition and a very poorly running engine. With no codes, and a signal that looks normal, all he can do is install a new sensor to try it.
Compounding that is the computer learns the characteristics of the various sensors. Once one of them fails, the computer can stop looking at its signal, and "inject" an approximate signal to run on based on the signals from other sensors. For example, the throttle position sensor has relatively little say in how much fuel is needed by the engine so if you unplug it, the computer still knows engine speed and load, and can guess pretty closely how much fuel to deliver. It's when movement of the throttle takes place that the computer has no way of knowing that happened until engine speed catches up. That will result in a hesitation or stumble. Now, ... If you replace the MAP sensor in a misguided attempt at solving the stumble, the computer is going to be confused since no two sensors are ever exactly the same. Next, you figure out the throttle position sensor is defective, (or unplugged), and you take care of that. Of course there would have been a code related to that, and now that you erased it and the new part is sending the correct signal, the engine should be running properly, except the computer hasn't learned that the "personality" of the new MAP sensor has changed. Relearning the new characteristics can take anywhere from a minute or two to a few miles of driving after the engine has warmed up. After a few miles it might run perfectly, so do you assume both new sensors were needed? It could have run poorly at first, and if you didn't drive it long enough for the relearn to take place, you could incorrectly assume the new throttle position sensor didn't solve the problem.
This type of relearn problem is actually more common with temperature sensors but while they have a pretty big effect on tail pipe emissions, they don't really cause much in the way of driveability problems that we can feel. If a new intake air temperature sensor, for example, constantly tells the computer the air is ten degrees colder than it really is, the computer doesn't relearn the voltage signals from that sensor. Instead it relearns that based on that information, it is always giving the engine a little too much fuel, and it is constantly having to cut back a little from the base "fuel trim numbers, so it updates those numbers and creates a new starting point from which to begin calculating how much fuel is needed.
Okay, I realize that isn't exactly answering your question, and I don't have a better answer specific to the two parts they sold you, but it might help to look at it from a different perspective. When you are paying the bill, the mechanic has to be concerned with cost, time, and satisfaction. They don't get a commission on parts and they are the ones who determine which parts go into your car, not the parts department people. The parts department people are more than happy to sell more parts, but they have no clue what your car needs or how the mechanic is making his determination, so they just dig out what is asked for. Neither party is interested in pushing unneeded parts onto you, even though it might seem that way. The mechanic is more worried about his reputation, and the things that improves it are if you get your car back with a bill lower than expected, and you get it back fixed right the first time. Most shops pay their mechanics on the "flat rate" system where most common jobs have a predetermined time limit. If he beats that time, he gets paid the same amount for the job but he can move on to the next car sooner. Investing in advanced training and tools, and experience play important roles in beating the published times. The checks and balances comes in when he hurries too much and makes a mistake. Then he gets to fix it later for free, regardless how long it takes. He doesn't get paid again and he can't move on to another car, so he loses twice. The confusion comes in when you have to decide is the current problem the original one he didn't fix correctly, is it a continuation of the first problem that he didn't fix everything, or is it an entirely new, unrelated problem? They have a right to charge you all over if it's a new problem, but how do you explain that to the car owner when the symptoms are the same as when you had the original problem? I used to run into that when doing alignments. Car pulls left due to tire wear patterns. Rotate the tires, pull is gone for a month, then comes back. If the problem is tire wear again, but it's due to incorrect alignment, is it a new problem, or did I not fix it right the first time because I didn't recommend an alignment? What if it's a Ford front-wheel-drive car that comes from the factory with REAL bad front alignment to make them ride smoother than other car brands, but that alignment can't be adjusted? Now who's fault is it?
What I was starting to get to before I got side-tracked, again, is when your car has a problem similar to yours, and it's under warranty, even more parts would have been replaced than the two you were charged for. I worked for a very nice Chrysler dealership but most manufacturers have similar policies. With a problem with a Chrysler product under warranty, there is very little argument over which parts get replaced. If the mechanic asks for it, it goes on the car and Chrysler pays for it by supplying the part. The first thing they look at is the car must be returned to the exact same condition as when it was new. The second thing they look at is if they've been seeing similar repairs more frequently than normal. When they do, they want the old parts back to see why they failed, then they go back to the supplier and insist on improvements or modifications. If they see that a few people fixed a problem that has a lot of other mechanics stumped, they'll issue a service bulletin to make it easier for the next people when they run into that problem. The manufacturer doesn't track the time on most jobs because everyone already knows how much they're going to pay for each procedure. That's for things that have set times such as "replace water pump" and "replace yaw sensor". They have different ways of tracking and controlling time on the job for unusual things such as locate the break in a wire, or "locate the cause of a misfire".
Since the manufacturer supplies all parts for cars in warranty, everyone is better off when the mechanic leaves in all the parts he tried, within reason, of course. On many GM cars, the anti-lock brake front wheel speed sensor can be rather time-consuming to replace since it's built into the wheel bearing assembly. If the new one doesn't solve the problem, it can take an hour to remove it and put the old one back in. If he put the new one in per instructions from the manufacturer's warranty department or help assistance line, they will have to pay the time to put the old bearing back in. The labor cost will be higher than their cost of leaving that new bearing in so there won't be any argument when the warranty claim lists the wheel bearing AND the part that fixed the car. It's not uncommon to have three or four parts billed on that warranty claim.
It's different though when the car is out-of-warranty. Now it's not a $50.00 bearing vs. $80.00 labor. It's a $250.00 bearing vs. $100.00 labor. (I'm making up the parts costs; I'm sure they cost the manufacturer more than 50 bucks). Even though a mistake might have been made in the diagnosis, the mechanic knows you will be money ahead if he puts your old bearing back in. That would be what most of them would do, except when they know the history of that part, referred to as "pattern failures". Every manufacturer has them with different parts. Those are the ones that show up on internet searches and you find out everyone is complaining about the same things. In the case of those GM car front wheel bearings, it is real well-known that they develop a little play and looseness, which is perfectly normal and common, but because they build in the anti-lock brake wheel speed sensor, which isn't much to brag about as far as wheel speed sensors go, that play allows the speed signal to drop out intermittently. Every GM dealer's scrap metal bin is full of those bearings. Problems can show up in as little as 15,000 - 20,000 miles. Now, ... If you had a car that appeared to have a bearing that was causing the problem, and it was rusty on the outside meaning it has been on the car quite a while, ... And a new one didn't solve the problem, what do you do? Sure, you continue diagnosing until you find a different suspect, and it solves the problem, but what about that bearing with the very high failure rate? Do you put the old one back in to save the customer some money, or do you leave it in knowing the customer is going to have another problem in a few months? Or, do you try to explain this situation to them and let them decide. You know they don't have enough knowledge and experience to make an educated decision, so they're going to be like me and let their wallet rule. Look at how much I typed here, then imagine a mechanic, who typically has poor communication skills with customers, trying to explain why he put two parts on their car.
I'm sorry I didn't do a better job of answering your question. I left the dealership before yaw sensors were out but I do know they aren't the simple sensors I'm used to. They respond to tilting, turning, and speed changes of the car, and include a pile of computer circuitry that also looks at individual wheel speeds. My electrical experience goes back to tvs and vcrs, and I saw over and over how unreliable things get when they become more complicated. Automotive computers use computer-grade components which are much higher quality than those found in consumer electronics, but it still fills me with skepticism, especially when I see how expensive replacement computers are and how difficult GM makes them to program to their vehicles before they will work.
In most states you are allowed to have old parts returned unless you allow them to be thrown away. Most mechanics will still leave them on their bench for a few days in case you come back looking for them. Computer modules are the exception, and your yaw sensor might fall in that category. Most computers can be repaired and updated, then sold as "remanufactured" so your old one goes back for the "core charge". They are usually better than new because besides other repairs, all common failure parts are replaced too. They cost about half as much as a new computer. Unless you were told otherwise, I would suspect if that sensor is a computer, you probably got a remanufactured one. I tried to find a picture of one, and apparently they aren't too popular as in need to be replaced often. Also, some look like expensive computers that can be rebuilt and some look like inexpensive sensors that are just thrown away.
If a mechanic isn't busy, he will often take the time to point out what's wrong with a part, (it's an ego thing). We get to show off our knowledge), but remember what I said about communication skills. We talked about that a lot in class but just like doctors talk with other doctors differently than they do with patients, mechanics understand each other but they aren't good at putting things into words customers can understand. Not understanding breeds mistrust when nothing was done to deserve it. Now add in a service adviser and look out. All new car dealerships and many independent shops have them. THEY are the people with good customer communication skills but they very often know little more about cars than their owners do. They can easily get confused when getting information from the mechanic, then they have to translate what they got wrong to you and hope you don't also hear something different than what they said. I think that's what they mean by "too many cooks spoil the soup". Now you have two opportunities to give you incorrect information when there was no intention to deceive.
While searching all over, I came across this site that I'm going to peruse some more later. If you look at some of the posts, many of them state that multiple parts were replaced in an attempt to solve a problem. First of all, I hope your problem is fixed. Then, this dissertation should at least help explain why they might have had to put on more than one part. If you'd really like more examples, I can give you a whole bunch of good ones related to suspension and alignment, and parts you can't tell are worn out until time-consuming disassembly is done to replace other parts first.
Wednesday, February 29th, 2012 AT 10:15 AM