The automobile of today is an incredibly complex machine. Through the extensive integration of computers, this complexity can be utilized and made largely invisible to the driver. This is by design and a wonderful feature of modern vehicles. Simply turn a key an it starts, push a button and the climate is controlled, drive 500 miles without effort, until something goes wrong.
Depending on how components are counted, a vehicle may be made of 500,000 or more. For instance, is a radio one component, or the thousand pieces that make it up? As any piece can cause failure, from a repair standpoint, the later count applies. Any component can cause a problem. Knowing this, manufacturers have attempted to add diagnostic capabilities to their vehicles.
Most post 1996 vehicles can track and record up to about 2000 different conditions. When a fault that can be tracked occurs, the computer operating the system attempts to log a diagnostic trouble code (DTC.) This is a combination of a letter and numbers giving a clue as to what has occurred.
Contrary to common belief, a DTC does not tell what is wrong it is an interpretation of data, and not data in itself
A common example of a DTC might be P0304. The P indicates the fault was found by the power control module (PCM.) This is the computer that operates the engine and sometimes transmission of the vehicle. The 03 indicates this is an engine misfire and the 04 indicates cylinder number four is involved.
This may seem like a lot of information but it must be remembered that dozens of things might cause a misfire of cylinder number four. A bad spark plug, plug wire, a vacuum leak, a bad injector, stuck or bent valve just to name a few.
A trained technician must apply knowledge and several instruments to isolate the actual cause of the DTC. Making things more complex, it can also be a false code. For instance a bad sensor might set this code even though no misfire exist. A problem could also exist on a companion cylinder and manifest on cylinder four. For instance many engines use a single coil to fire multiple cylinders. A problem in the first cylinder may cause a false DTC to be set in the second.
There are many other situations which can provide equally misleading diagnostic trouble codes. Recently a vehicle came into the shop with a code stating exhaust gas recycle (EGR) valve fault. The EGR valve had been previously replaced by the owner, but the code remained. The owner then replaced several other expensive parts, in a vain attempt to fix the problem.
The actual problem was found to be ground wires between the fuel pump and the fuel gauge sender unit. The wires were inadvertently crossed in a previous repair. Because both are grounds, the vehicle ran, but the fuel sender unit wire returns to the PCM. This caused the vehicle to set a false EGR code and the client to spend a great deal of money.
This type of scenario plays out all too often. Some part stores are eager to check codes for free. Why not, they sell a lot of parts and hold no responsibility. This type of guess work is far too expensive for the average motorist to afford. Worse, substandard parts can actually create new problems that did not previously exist. There is no substitute for a well trained, knowledgeable professional. It’s simply much less expensive to test than to guess. AGCO, it's the place to go!
