What’s a fair mechanic diagnostic fee

Mechanic diagnostic fee

If your check engine light is on and you take it to a shop, they’ll charge you a mechanic diagnostic fee. Many times it can be as much as $145. Yet auto parts stores will read the trouble codes for free. So why do shops charge so much if it’s as simple as plugging in a code reader?

First, auto parts store use a $39 code reader and those scan tools can’t read anything other than generic powertrain codes. That means they can’t read manufacturer specific codes, body “B”, chassis “C” or data bus “U” codes. Professional technicians, on the other hand, use a $3,000 scan tool that can read all codes. Secondly, real mechanics get the code and then go on to do further diagnostics to confirm or deny what the trouble code says. Here’s an example.

Let’s say an auto parts store reads the trouble codes and their reader shows P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I). The auto parts store will immediately recommend replacing the oxygen sensor. After all, the words oxygen sensor are listed in the code, so that must mean it’s bad, right? WRONG.

If the engine has a vacuum leak, the extra air could be affecting the air/fuel mixture. Here’s what happens when you have a vacuum leak. First, the oxygen sensor starts seeing a lean condition in the exhaust due to too much air entering the system after the PCM has gotten an incoming air reading from the MAF sensor. To correct the lean condition, the PCM will add more fuel to balance. But there’s a limit to how much fuel the PCM can add.

A real mechanic would see the P0131 code and immediately switch over to live data and start reading the short and long-term fuel trim readings. That’s what the PCM calls the process of adding or subtracting fuel. If the mechanic sees that the PCM has reached the maximum amount of added fuel, yet the oxygen sensor still reads P0131, they’ll suspect the sensor is good and do additional tests to confirm or deny a vacuum leak.

In addition to checking other engine parameters through the live data screen, they’ll also look up any manufacturers’ technical service bulletins to see if there’s a pattern failure with this type of engine.

So you see, the mechanic diagnostic fee is far more than simply throwing a code reader onto your car. In fact, a mechanic diagnostic fee is more like a doctor ordering tests and then interpreting the results to see if more testing is in order. You have a cough. The doctor orders a chest X-ray. The radiologist report says your lungs appear congested. The doctor doesn’t order a lung transplant, he orders more tests.

In car repair, an auto parts store clerk or an inept mechanic simply replaces the part listed in the trouble code (like a lung transplant). A skilled mechanic does further testing.

Skilled mechanics attend about ½-dozen classes per year to keep up with the latest diagnostic techniques. They read all the service bulletins that come out. And they rely of very expensive diagnostic equipment.

Here’s what a real mechanic’s diagnostic fee covers

Getting the trouble code is just the first step in the process. If you want to see what goes on AFTER the tech gets the code, watch this video. Think about the full hour the tech spent finding the actual cause of the problem. If he had relied on just the trouble code and replaced the barometric (MAP) sensor, he wouldn’t have fixed the problem!!!!

Replacing the part listed in the trouble code without performing further diagnostic work is the single biggest mistake made by DIYers and hack technicians. Read diagnostics takes time and you have to pay for it.

So quit your bitching when they charge a diagnostic fee. They earn every penny of it.

©, 2014 Rick Muscoplat