Why do mechanics, heedlessly, change sensors?

It’s heard, time and time again, “The DTC code showed a problem in a sensor circuitry. The mechanic changed the sensor. It didn’t fix the problem! What is the problem?”

What IS the problem with mechanics who change these expensive sensors on NO evidence? What do you think are the “reasons”?

The main reason is they don’t know what they are doing.

Every DTC has a diagnostic chart to follow to help determine what the cause is. Most times the cause is not the sensor or part described by the DTC. Most times it’s a poor power or ground connection. The diagnostic chart will give you step by step directions to check all the related components first before it tells you to replace the part.


I spent some time during my youth ('70s/early 80s) not afraid to mess with cars and I would work on my own. Then came things like engine computers, emissions systems, fuel injection, front wheel drive etc. and the “common wisdom” among everyday folk that I knew was that cars these days are just too complex - computers and specialized tools and diagnostics and no room to work under the hood. “Don’t mess with it” was the wisdom.

So at some point I stopped messing with cars and ignored mine, not really understanding them.

Then, partly out of necessity I started messing with them again and didn’t find them nearly as mysterious as everyone had told me in my teen years.

But here is where I am going - and it is a stretch - but why not? The key mechanics in many shops are likely my age and older. Many of these were probably trained largely by “old timers” who cut their own teeth on older vehicles with things like carburetors and lack of computers. I think there is a whole generation of mechanics (obviously not all! no offense to anyone!) who are sort of working on a hybrid logic of old cars and new systems. Faced with the “new-fangled” there is head scratching, shrugging and parts throwing. It just used to be so much easier to pinpoint things.

Of course, I could be completely wrong about that. Less of a stretch - parts stores want to sell new unneeded parts b/c selling parts is how they make money. Well, mechanics also make money selling parts and likely make a lot more, e.g. replacing a sensor than a vacuum hose. How much markup can you put on a $.25 piece of hose?

The world is full of boat payments to make!

Reason number one why an electric car isn’t good for the economy in the eyes of all those mechanics and part suppliers.

I can only offer opinion and it is that speed (or how many jobs you can put out) is so strongly emphasied by everyone in the repair chain.

You get a car with a 'check engine" light on and your inital look over does not reveal the answer, so you go the the Service Advisor and tell him you need more time. The advisor does not want to call the customer with this information and in addition he is thinking of the previous 5 check engine light jobs that he dispatched to a different mechanic, and the jobs were resolved without the request for more diagnostic time. It just so happens that you will get lucky and make a correct guess quite a bit of the time.

There could be another underlying reason and consider this.
A customer coaxs a poorly running/multiple codes/CEL on/whatever vehicle with 80k miles on it into a shop. Usually the shop or mechanic knows nothing about the history of the vehicle.

So if one assumes that the proper basic diagnostic steps are to be taken this means starting with a vacuum test, compression and or leakdown test, scanning codes, fuel pressure test, possible examination with an oscilloscope, etc, etc. etc.

This is going to take time and each operation will, or should, require a charge to the customer.
What will a customer say when told up front that they are going to be charged for performing a number of tests? The majority are going to balk or scream bloody murder and will have the attitude of the gentleman who wrote a letter to the local paper here a few years back in which he stated that “any idiot can be a mechanic nowadays. All you have to to is have the computer tell you what part to replace”.

Too bad it’s not that easy. And I do agree that some mechanics also have that “computer says” attitude.

On the other hand, I have seen certain electronic areas in which factory service manuals will flat out state that “if all tests show the system is good, then try replacing xxxxx”; whatever that part may be.
I’ve run into things like this several times in which every single part of the problematic system was replaced with brand new factory OEM parts, all wiring tests fine with a VOM, and yet the system was still inoperative. What’s a tech supposed to do then?

The real problem is a lack of diagnostics skills, and tools to make the correct choice.

Since a typical shop isn’t going to have every possible sensor and component in stock to be able to replace the parts as needed until they find the actual problem, the shop just takes a guess based on what the CEL code reads.

If they get a code that tells them the long term fuel trim is too lean, then they will typically start with the O2 sensors. That typically isn’t the cause, however.

What they should have done is hooked up a data logger, and captured the data as the car is being revved, and compare the MAF sensor values to see if they are in spec. If the MAF sensor tells the computer that it is flowing more air than it actually is, the O2 sensors will tell the computer that the engine is running lean, and will constantly richen up the mixture, until it can’t anymore. Then it sets the CEL.

The code tells the mechanic that the O2 sensor says the car is out of spec, he scratches his head, and slaps in a new set of O2 sensors, and the problem isn’t solved.

A couple of minutes with a data logger, and recording the MAF values at idle and at 2500 rpms should provide enough data to let the mechanic know if the MAF sensor is in spec or not when compared to what the range should be.


I agree with everyone that there are multiple factors which result, at times, in less than desirable outcomes in emissions systems repair. It doesn’t help that customers, in their layman’s minds, understand “parts changed” much better than they will accept “diagnostic time”.
I see ignorance, as part of the problem. Ignorance can be cured with study and learning. I don’t think that enough effort, by shop owners/managers, and individual mechanics, is put into curing the ignorance. How many mechanics understand that they are dealing with systems, not just parts; and, that those systems are part of the whole? An elephant is more than a leg here, a trunk there, and a tail there. Those are only parts of the elephant (the whole).
EFFICIENT diagnostic techniques aren’t available, or taught, if available. Wandering through a diagnosis isn’t efficient. We need more people trying to develop these improved techniques, and mechanics applying them.

On the other hand, I want action. I took my car in last week because it had intermittantly stalled on me three times that day. I wanted the guy to check the pump, check the coil, check the ignition switch, check the crank sensor. And if he couldn’t find anything, start replacing the most likely problem, the crank sensor. Got it back, couldn’t find anything so cleaned the throttle plate etc. Said to see if it did it again. Well its a big pain to substitute cars and take it in, so I want them to take their best guess. I don’t want to be out on the highway and find out it still does it. So now I’m running it around town trying to see if it does it again, but I would much rather have paid the extra $100 to just replace the dang sensor and if that wasn’t it, we’ll go to the next level.

I have full understanding that sometimes it is near impossible to diagnose a problem precisely, especially intermittants, so parts replacement and substitution can sometimes be the cheapest and quickest way to go.

Do you mean needlessly?

Important distinction.

I agree that a lot of mechanics will go with the “most likely” cause and replace the first thing that comes to mind. A mechanic needs experience with a lot of different problems to make a correct diagnosis. I have found over the years that problems that look like 02 sensors turned out to be the following: Plugged air filter, torn air filter, cracked vacuum line, missing vacuum line, plugged vacuum line, bad gas and plugged muffler. Anything that will affect the richness or leaness of an engine will drop a code that looks like an 02 sensor fault.

I think it has do more with expierience when a DTC comes up and what the most likely cause is.

If a DTC comes up P0133, it’s most likely a problem with O2 sensor than the O2 sensor circuit itself. And to completely test the O2 sensor circuit itself would cost many times more as just replacing the sensor.

But if you get a DTC P0300, that’s a different story. There are many things that can contribute to this DTC showing up. And that’s when a diagnostics must be performed. And because of this, that’s when things can get expensive.


You could fault techs and customer alike.

  1. Some techs are simply parts replacers.

  2. Some customers do not give proper or complete information to a problem, therefore the tech is condemned to making an improper repair.

For the short time I was a service writer I always preferred dealing with women drivers since the information concerning a problem was typically more complete from them. On the other hand the info from a male driver was vague or incomplete. I loved it when a guy came in and explained the issue and we would give an idea of what it could be and the guy would say “that’s not it”. After 3 or 4 times of this it was like, why did you bring your car in since you know more than us, after all you can’t fix it since you brought it here…sheeesh.

No one has mentioned the manufacturers who design and build this junk…There are over a thousand DTC codes. Many of them lead down dead-end rat-holes. Consumers won’t put up with this stuff forever…There is a limit on how many times they will spend $1000 or more to make a nag light go out…

Skilled, competent mechanics are getting very, very hard to find. Our educational system simply does not move bright people in that direction. Today’s “sensor changers” were yesterdays “Lube Technicians”…

A sensor that has been in a car for close to 100K miles and 5 to 10 years has been subjected to heat, moisture, dirt, and whatever else is involved in the operating enviornment for many multiple cycles. They can wear out, clog up, and break down. Replacing the sensor might not be so needless, sensors do not have an “unlimited” life. History of the car (has the same sensor ever been replaced before), miles driven, and knowledge of the brand and particular sensor could make a bad sensor a most likely correct diagnosis.

Are there random “parts changers” out there with no diagnositic skills? Sure, but there are also excellent mechanics that will diagnose a “bad sensor” immediately and then do the proper checks to confirm it before changing it out.

I think this thread highlights the value in finding a good honest mechanic and using that shop for all your maintenance/repair needs. My mechanic has told me that that is the biggest problem he has, customers who do not give “proper or complete information”. I always compose a detailed description of the problem in a word document and print it out and attach it to the repair request as opposed to handwriting it out at the garage. I learned the hard way not to second guess the mechanic, because I am usually wrong.

Also, I won’t go to a “chain” shop anymore. I have seen and read too many times in the news about how the mechanics work on commission, so the incentive is for them to replace parts needlessly.

My theory is, the chain shops don’t worry too much about losing a customer because they are always advertising on TV and in print and they are always pulling in new customers. On the other hand, the garage I go to, I speak directly to the owner and his wife, and he has a strong incentive to keep his customers happy because he does not have the large advertising budget the chains have. Of course, this is only my theory and you can feel free to disagree.

Why do mechanics, heedlessly, change sensors?

Mechanics don’t. Employees of the quick oil change places do.

I don’t think it’s a “black or white” situation. As others have mentioned, often times changing a part for a given set of symptoms and seeing of it works is cheaper than doing in-depth diagnostics, especially when the parts change is backed with underlying knowledge and experience, and most of the time it works. And, as OK4450’s last paragraph in his post pointed out, sometimes the manual will simply say to change the part…and it usually works.

The ones I have a problem with are the posts where a shop has repeated changed the same expensive part or assembly or keeps changing one part after the other with the same unsuccessful result and lacks the expertise and/or integrity to break out the manual and dig deeper.

I Think Replacing A Sensor Without Thoroughly Testing The Circuitry Is A Function Of The Cost Of The Sensor, Ease Of Replacing It, How Much Time You’ve Got, How Cold It Is Ouside, How much Beer You’ve Got, And Sometimes Miles On Car & Age Of Car.

I once was guilty of putting an oil pressure sensor on our old, high miles 350 Omega because the pressure light was flickering at idle. Six bucks, five minutes, cold hands, two beers, and Voil? ! Lucky guess, it was fixed!

My wife’s car with fairly high miles, several years old, had “check engine” light and code for bank 1, sensor 1, O2 sensor. It was overdue for new ones (replacement is recommended but, I don’t routinely replace them). That sensor was right there, the world’s easiest location. It was less than $50, the wiring looked OK, it was very cold out, I didn’t have much time and Voil? ! Lucky guess, it was fixed!

My son’s car had an intermittent “stumble” and code for MAF sensor. The code didn’t fit the symptoms. It was very easy to get to (2 minutes), we didn’t have much time, my son had the $150 aftermarket sensor. We cleaned the old one and it didn’t fix it. I told him to return the sensor to the store until we could thoroughly diagnose the circuit, check voltage aginst the service manual, and check over all electrical connection (wiggle test, etc.).

I fixed several intermittent electronic “ghosts” by “educated guessing” at a body control module. I can’t get BCM codes out. The salvage yard sold me one for $25 and would let me return it if it didn’t fix my problem. It did.

I fixed a cruise control the same way for $5 by popping in an easy to install control module, otherwise I probably would have let it slide. It was an old, high miles car.

I have a super salvage yard near me.
For me, each situation has to be evaluated for a number of factors.