Misconceptions about Scan Tools
Many folks think that modern car diagnostics should be a snap. Just hook up a code reader to the port under the steering wheel and it tells you which part to replace to fix the car.
That’s it right? Even a monkey could do it. What is the big deal?
I am sorry to report that nothing could be further from the truth. The reality is that modern internal combustion engines are a marvel of computer engineering and thermodynamic and hydrodynamic wizardry. There are usually computers on board just to control the fuel injection. Computers exist for the body control, power train control, HVAC, security and so much more. Feeding information to those computers are modules and under those modules are the actual sensors. Now to make it even more complicated you could have malfunctioning sensors, other modules in the system that are feeding bad information to the sensor reporting the code and failed parts that are linked to other systems, broken wires, melted relays, and a myriad of other problems that can just confound diagnostics. There are even soft codes and hard codes. A soft code is something the computer has only seen once or twice. In other words it is an intermittent problem. A hard code is something that the computer sees consistently.
Not all codes refer to a single part. Codes such as the famous P0300 Random Misfire Code are the bane of diagnosticians everywhere. The P0420 Catalytic Converter Below Threshold Efficiency can be caused by bad gas or bad Oxygen Sensors or a bad Catalytic Converter.
Yes the On Board Diagnostic System, which is usually referred to as OBDII, provides a wealth of information that any seasoned diagnostician needs. At the same time there is an old saying that goes,
“Drink deeply from the well of knowledge or drink ye not at all”
This means that a little knowledge can be a dangerous thing. These parts houses that pull codes for free and then sell you parts based on those codes are quite often diagnosing problems incorrectly. Parts houses sell parts. Repair facilities repair cars. And any reputable repair facility will pull the codes for you free of charge. And then, if possible, help you interpret what they mean.
If the code gives the technician enough information to diagnose then we are home free. An example would be the Gross Evaporative Leak Detected code. Usually means a loose or worn gas cap. Quite often the code simply tells the technician where to begin looking. In that case, paying the doctor to make the correct diagnosis can save a lot of wallet pain before he/she starts performing surgery on your car.
I am only writing about this to help correct some commonly held misconceptions that are costing people more money and time than they usually have. Beware of laymen diagnosing your car. Skilled technicians spend 10s of thousands of dollars and years learning how all those electronics work and even more than that on the tools of the trade. Charlatans offer help without the knowledge to give it.