Why would you pay to diagnose a check engine light?

Why should you be willing to pay a mechanic to diagnose your check engine light? We’ve all heard commercials from stores like Pep Boys and various tune-up shops for “free check engine light diagnosis” — even the transmission chain Aamco, with no general repair track record, is advertising “free” diagnosis of check engine lights.

If advertising were considered a reliable guide, one might assume it must be pretty easy to “hook up” and have “the computer” tell you what’s wrong with a car. Why would you want to pay for something you can get for free?

The question is — What kind of a diagnosis do you get for your zero dollars? Is it of any value?

Or is it possible that choosing to go the “free” route will end up costing you more than paying a proffessional to truly diagnose your problem. Let’s take a look at some of the components critical to a successful diagnosis and repair:

  • The diagnostic process used
  • The technician’s knowledge
  • The equipment available
  • The quality of the repair parts

The diagnostic process

Most of the time a “free diagnosis” will include no more than pulling trouble codes. Which means a scanner is connected with your vehicle and self diagnostic results, known as trouble codes, are retrieved. This only take a few moments, and we frequently do this for customers with no charge.

However, we never represent pulling codes as a diagnosis! We might check codes for a customer if the check engine light comes on just before they leave for a trip to assuage their fears, or if a customer thinks he may have left the gas gap loose.

A trouble code DOES NOT indicate what is wrong with the car — it’s just one clue to point the technician in the right direction. A trouble code may point to a sensor, but that does not always mean the sensor is bad! In fact, depending on the year, model, and trouble code, there may be very little chance the trouble code is telling the mechanic to change the right part. Any mechanic who remembers OBDI Hondas could attest to this after replacing an oxygen sensor after finding a code 1, or replacing a fuel injector after finding a code 45. (In both cases there is a better than 90% chance the car will not be fixed by replacing the part the computer “thinks” is bad).

And what will a tune-up shop do about codes that do not point to a specific sensor, such as the fairly common P030X misfire codes? You’ll most likely get a guess based on the code, followed by something like this — “Well Mr. Jones, your car needs a tune-up. We should do that first and see if it takes care of the misfire code”.

The thing is, sometimes this type of approach is valid after some testing and logical deduction. However, we’ve had cars come in from other shops with problems like this —

  • P0301 code (cylinder #1 misfire)
  • A dead misfire only at idle
  • A new set of crappy tune parts,
  • And an obvious leak at the #1 intake runner.

Any qualified tech would have known that ignition parts would not fix the misfire based on a test drive alone, yet the customer was charged for a tune-up without apology and advised to go to the dealer when they realized they were unable to diagnose and repair the problem.

If this customer had come to us for a paid diagnosis, he would have —

  • paid much less,
  • known what was really wrong with his car,
  • and still possessed his original high quality ignition parts instead of the junk the other shop installed.

And having a set of unneeded tune parts installed is probably one of the least costly errors “diagnosis by code” is likely to cause. How about paying $2000 for a new catalytic converter to cure a P0420 (converter efficiency) code when all you really needed was an exhaust gasket or O2 sensor?

Diagnosis is not pulling codes; it is using either manufacturer supplied test procedures or industry accepted test procedures to find and verify the root problem. Pulling codes then installing parts is neither manufacturer approved or industry approved.
The technician

A mechanic who has put in the effort master his trade does not come cheap, at least not cheap enough for the chains & tune-up shops. Two years of school coupled with at least two years of on-the-job training is what’s necessary to forge a green mechanic, who is still learning and making quite a few mistakes, but is able to pass all 8 ASE tests as well as the L1 advanced diagnostics test.

Even at this early of his career, a technician is unlikely to work at a tune-up shop, at least not for very long, because so many better opportunities exist. As a technician matures through experience and continuing education, he is even less likely to be found in a Pep Boys, but he is more likely to be able to find and fix your problem on the first try. In short, technicians who can diagnose problems correctly the first time are fairly well paid. Tune up shops and chains do not pay technicians well, despite charging high labor rates and obscene markups on low quality parts.

The equipment

OBDII is a mandated standard for on-board self diagnostic systems in cars. It’s a wonderful thing because it ensures that at least some diagnostic information is available in a standard format that can be accessed using a low cost generic OBDII scanner. These scanners can cost as little as $35.

I use a generic scanner daily, but only for two things: clearing codes after repair, and checking monitor status. Why not use it for diagnosis? Because it displays less than half of the available information! On some cars, like the Prius, it displays less than 10% of the information! Also, generic scanners lack bi-directional control which allow time-saving tests that can save hours of diagnostic time during a proper diagnosis.

If you want all of the information the car has to offer (and you do if you want to accurately diagnose cars quickly), you must buy the OE scanner for that make. OE scanners range in price quite a bit. Some of the scanners we own cost as much as $7000 plus yearly software subscriptions. We currently have about $25,000 in scanners at our shop, and that doesn’t even take into account software subscriptions or upgrading as they become obsolete.  Is it worth it?  Absolutely, for us AND our customers. Having the right equipment saves a lot of time and increased accuracy which means lower bills and fewer repeat visits.

The repair parts

Let’s assume a shop that offers “free diagnosis” has a good technician, adequate equipment, and spends the time to properly diagnose the car. Does it sound like a good deal now that it looks more like you might be really getting something for nothing? Not so fast! What about the parts used to repair your car?

Time/money lost on free diagnosis can be recovered in parts mark-up. For instance: I can buy a wholesale catalytic converter for as little as $50. Genuine original equipment converters can cost as much as $1800. Might a shop engaging in a dishonest advertising stratagy be willing to install an extremely low quality inexpensive part in your car, and bill you as if they installed a quality part?  Yup!

I have seen many examples of this, most often when I am telling a customer a shiny new part on their car is bad. They say, “but it can’t be, I just had it replaced!” Usually followed with an invoice from another shop with grossly overpriced parts. Sometimes the parts are billed at well over dealer list price, yet the parts installed were the lowest quality aftermarket crap available.

The moral of the story: you get what you pay for

In order to properly and correctly diagnose the wide array of problems that will cause the check engine icon to illuminate, a technician must use his knowledge and experience of the systems involved, and follow manufacturer recommended diagnostic procedures and industry standard practices. This will take time — sometimes considerable time.

It is simply not possible to consistently fix cars when a marginally skilled technician uses substandard diagnostic techniques and hobbyist-grade equipment to “diagnose” the problem, then “repairs” the car using high-margin low quality parts. If even one of the key ingredients is missing, the outcome will likely be poor.

  • A shop that pulls codes then guesses will not come up with the correct diagnosis most of the time. They surely know this, and are likely to use slippery wording and “recommendations” rather than properly document the original customer complaint, tests performed and results, and the repair recommended to fix the original complaint (as opposed to “a recommendation”). Without documentation, good luck getting your money back if the car is not fixed.
  • A shop that uses low-quality/high-profit parts to recapture money lost on time spent during “free” diagosis, will produce poor quality repairs. Even if they have a good techs, techniques, and equipment, your car won’t be reliable after repair.
  • A shop that uses inexperienced or ignorant techs to reduce overhead will not be able to properly diagnose cars, even if they have the best equipment and use the best parts.
  • A shop that uses generic scanners for diagnosis will struggle with some problems and will be simply unable to deal with others. I assert that the time wasted trying to “do without” will far exceed the cost of proper equipment. As a consumer, you pay for business costs, whether you believe it or not.

When I’ve asked people, “How do you think these shops pay the rent, payroll, and other expenses if they are doing work for free?” I get one of two responses:

1. Diagnosis is easy. You just hook up the computer, so there’s no reason to charge for it.
I say — No. Guessing is easy; diagnosis is time consuming and requires knowledge and equipment.


2. They make the money back because they get to do the resultant repairs.

And I have to agree — They sure do! They lured you into the shop with a lie. What kind of value are the liars likely to provide?

There is no free lunch. Why would you want to pay for diagnosis? Because you want your car to be reliable.