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Charge for a Diagnostic Engine Scan for a Check Engine Light After Code Already Known

Why do repair shops insist on charging $90 to $100 for a diagnostic engine scan even after you supply the code which can be obtained at most auto parts stores free of charge?

My 2005 Toyota Prius was indicating a P1121 diagnostic code which I obtained from a free Autozone scan. My repair shop insisted that they also perform the scan for $90 to “pinpoint the problem”. Was this really necessary?

Yes it was neccessary.
The ‘‘free scan’’ may only have picked up one of many codes or the tester there might not have the tech sense to know there’s more to the picture when doing the '‘free scan.
Most shops I’ve ever dealt with do not and will not just blindly believe someone elses ‘diagnosis’ without performing their own.
Few shops will ever blindly peform a repair diagnosed by someone else with out a disclaimer to cover their butts.
We get a lot of that here. ex; “cust states replace spark plugs” is all the repair order says. - 2 days later customer is back saying "you guys just fixed my truck but it still runs like crap.’‘
To which we must remind them ‘’ You never said to diagnose ‘runs like crap’…all you ever said was to replace the spark plugs.’'
And an argument ivariably ensues over who’s ‘‘fault’’ it is that the ‘repair’ didn’t fix their problem.

Then why such a high price for such a simple task? The scan devices themselves only cost anywhere from $50 - $200.

Same as going to a second doctor.
’‘Well Dr X, I was told by Dr A that I need a so-and-so.’’

Just what do think she’ll do ?

Most shops roll the diag charge into the final repair. If yours doesn’t, find one that does.
Yah, the thing with all the buttons only costs x dollars…but knowing which button to push ? priceless.

Your dealer may be using the wrong term, they need to perform a diagnostic test to identify the problem. Or based on your free scan you could tell them what part to replace.

The solution to your problem may be in this bulletin.

A mechanic in the shop may be using a tool that is far more expensive than a few hundred dollars and will delve deeper into the reason behind the problem.

The mechanics in a shop work on the flat rate pay system, which means they only get paid for what they actually do.
It would be extremely easy for a shop and mechanic to perform free scans and diagnose engine driveability problems all day long for free. The problem with that is that the banks want their mortgage payments and the grocery stores are not giving away food.
The shop has expenses to cover that you can’t even imagine and without those charges their doors will close.

And no sane shop should ever rely for one second on anything that a customer relates to them about a self-diagnosis or one that was given by some guy behind a parts counter and who is just a year out of high school.
Keep in mind that a code for a particular problem does not mean that particular part is faulty. That code is a starting point for a real diagnosis only. (E.G., an O2 sensor code does not mean the O2 sensor is bad, etc.)

It can vary with the shop. When my car was last in the shop (for inspection) I asked the service manager to also replace the EGR valve and solenoid. He told me there would be a diagnostic charge. I told him “I don’t want you to fix any problem. Just replace those parts.” He hassled me a bit but eventually relented. So he replaced the parts without a charge for diagnostics. And my lit CEL turned off.

Ken Green has explained why conscientious shops want to run diagnostics. But these shops also know full well that the charge is excessive. If a customer appears knowledgeable about his car, the mechanics will do exactly the service he asks, no extra charges. Stand your ground.

"Then why such a high price for such a simple task? The scan devices themselves only cost anywhere from $50 - $200. "

My scantool cost over $9,000 when I purchased it. It has other components on it to help in diagnosis such as a 4 channel labscope and component testers. I pay $2500.00 per year to update it. The cheap scantools most of the time will read only basic OBD II codes and not manufacturer specific codes like Ken said. Even when my brother calls me from his shop to check a transmission and he has a list of codes, I still run them myself before I can give an accurate diagnosis. I frequently get different results from his and he uses a cheap $600 scanner. I want every bit of information I can get before tearing down someones transmission. I dont like to go back in twice.


Allow me to emphasize that on top of all of these notes about the equipment and shop overhead and all of that an error code is NOT a diagnosis.

That one is my pet peeve of “car myths” ( and a great reason that everyone needs a course in auto care basics (

Any shop that replaces a part based on a self-diagnosis by the customer or based on a code provided by a parts counter person is not only misguided to put it politely but they’re running a huge risk of being the bad guy in a repair that did not turn out the way the customer thought it would.

Some people who provide a self-diagnosis are perfectly understanding if the shop does a repair based on a provided diagnosis. Many others are not so understanding; often cursing, pointing the finger, threatening to sue, along with the development of amnesia where they have completely forgotten saying anything at all about a diagnosis they provided.

Walk into a doctor’s office while suffering the flu, tell the doc you have the flu, and want a prescription for X medication and a shot to get the ball rolling. Think about how that self-diagnosis is going to work out along with the fee that doctor is going to charge after spending 3 minutes with you and even agreeing with your diagnosis.

I looked up that 1121 code and it’s a TPS sensor inconsistent with the MAF sensor. Since things are perceived to be so simple then fill in the blank.

To repair this problem I would ____________________________ and the problem wil be solved.

“To repair this problem I would ____________________________ and the problem wil be solved.”

…Pay $90 for the test or replace everything.

BTW TPS codes are P012X.
P1121 is “Coolant Flow Control Valve Position
Sensor Circuit Stuck”.

As many others have noted, a code scan is definitely not a diagnosis, and a proper diagnosis often requires better equipment than can be bought at the auto parts store. If you want to get started on being able to properly and accurately diagnose these problems yourself, get online and subscribe to ALLDATA, which is where most mechanics get their information on how to properly diagnose these problems (flow charts, specs, wiring and vacuum diagrams, etc). If you want info on all makes and models, that will probably run you a couple or few thousand dollars a month. One year, make, and model runs $40-50, I think. You will then notice that ALLDATA’s diagnostic procedures will likely tell you to connect a scantool and command various tests on sensors, solenoids, etc, so you will need a tool capable of doing that. To get one, call up your friendly Matco or Snap-On tool man and go on his truck to look at the scantools. If you’re lucky, he may have a used one that has been well cared for and just needs updated. The used device will run you several thousand dollars, and the updates to bring the device current will be another couple thousand or so. Here’s a good kit that will get you started, priced minus the most current updates:

The most important thing to remember, though, is that there is a vast difference between a diagnosis and pulling a trouble code. They are NOT one and the same. For your $90-100, that technician is doing a lot more than hooking up a $50 code reader and telling you what the code is. In fact, he or she may spend an hour or more working on your car and researching various things to arrive at a firm diagnosis. It’s not as simple as pulling a code that says “replace part x”. There is no such thing as a code that says “replace part x”.

A look at my book again shows the P1211 as a TPS problem. However, it’s a Chiltons and based on a Chiltons history it could be dead wrong. I’ll look at this as my being incorrect and admit it.

That does not take away from the fact that shop should never rely on a self-diagnosis from a customer.
I note that ALLDATA has 2 TSBs out on a P1211; one of them issued in 2006 and the other in 2008 on the '05 Prius. This usually points to a chronic problem that may have multiple causes.

That could be similar to Subaru’s driveability issue back in the 80s in which a TSB was issued every month or so for the same problem. A year later when the new models were being sent out there were half a dozen TSBs for the same complaint and for the same cause with half a dozen different fixes. The new cars were also rolling out of the factory with the same problems as the prior year’s model; as admitted to me personally by the Subaru factory rep who was plenty irritated with his new demonstrator.

The OP wants to take the car to the mechanic tell them the code from an auto parts store and just get some new parts thrown on as per the code. So, what if the check engine light is still on? Or, comes on a few days later? Does he go back to the chain store to complain that they didn’t get the code right or the mechanic and complain he didn’t fix the problem? If the OP is a doctor he doesn’t seem to value getting a “second opinion” before surgery.

Few shops will ever blindly peform a repair diagnosed by someone else with out a disclaimer to cover their butts.

Nor should they. Several years ago, I had a very intermittent failure in my Sienna, an evap leak failure. It would fail some times and not others, sometimes not fail for weeks.

I played with it for months. I knew if I took it to a mechanic, the most likely result would be a bill for diagnostics coupled with no trouble found, and do not wish to pay lots of money for no fix.

Finally, a man on the Sienna club had the same exact problem, but he had two Sienna’s. He swapped parts until he found it was the charcoal canister, there are some pressure operated valves there and he guessed there was a sticky one.

I took my car to Toyota, explained I was a retired technician, and was sure enough of my diagnosis I simply wanted the part replaced, no wasting my money in an impossible cause for an intermittent with little chance of seeing it fail.

They did indeed have me sign a waiver that I took responsibility for the diagnosis, which I was glad to sign.That, diagnosing embedded microprocessor problems, is what I did for a living for over 30 years… They replaced the canister assembly, and it has not so much as blinked since then.

When my brake computer showed a failure last year, I had to let them do the diagnosing because I had no clue.

I just remembered something that happened in the days of the VW fastback and squareback. An engineer had one, and it would really start and run hard when very cold.He would take it to the dealer, and they would let it sit in the warm shop so it would be nice to work on, and then report NTF.

He finally tired of this stupidity, and did some investigating on his own. There was a temp sensor which controlled fuel richness. He put a potentiometer on the dash, and had a manual choke, problem solved.

If you find a shop that begins to work on your car based on someone else’s code scan that you’ve passed onto him third-hand…run away. Go elsewhere. No reputable mechanic will do this.

If there is one thing I’ve learned in this business after 32 yrs. Its comunicate with your customer. Make sure you’re all on the same page, document everything on the R.O. and have them sign it. California’s “Write it Right” reg’s put out by the B.A.R. make it crystal clear and you are covered. I have no more problems explaining diag fee’s to people…they seem to understand it takes time and know how to fix todays cars. If you’re new to an area and need a good shop ask the parts guys at the local auto parts store or better yet the wholesale counter at your dealer who’s good…they know who’s throwing parts at a car and who’s fixing them. Who keeps returning parts until they get it right etc…

“I have no more problems explaining diag fee’s to people…they seem to understand it takes time and know how to fix todays cars.”

Your customers don’t sound like an average to me. These days people seem to always assume that computer “knows” what’s wrong - such as the Mad Dr - who is probably not coming back, btw.