Automobiles today are incredibly complex. Extensive integration of computers makes this complexity largely invisible to the driver. This is by design and a wonderful feature of modern vehicles. We turn a key or push a button and it starts. Another button controls the climate and we drive 500 miles without effort, until something goes wrong.
A vehicle may contain up to 500,000 parts, depending on how we count components. For instance, a radio could be one part, or it may be the hundreds of pieces that make it. From a repair standpoint, any component can result in a failure, so lots of opportunity exists for problems. Knowing this, manufacturers have attempted to add diagnostic capabilities to their vehicles. These are early warning systems that detect potential problems, and attempt to warn the driver.
OBDII diagnostic trouble codes
In 1996 onboard diagnostics II or OBDII became standard. Vehicles built after this date can track and record up to two-thousand different conditions. When a problem occurs, the computer that is operating the system attempts to log a diagnostic trouble code or DTC. This is a combination of a letter and numbers and gives a clue where the problem might be.
Interpretation of diagnostic trouble codes
A common example of a DTC might be P0304. The P in the first position means, the power control module, or PCM found the fault. This is the computer that operates the engine and sometimes transmission of the vehicle. A fault, which the body control module finds, will start with a B, and so on.
In our example, the character following the P is a zero. A zero in this position means the code is a Society of Automotive Engineers or SAE code. They standardize these codes, and they are the same among all vehicles. If the second number was a one, it would be a manufacture specific code. An example is P1301. Manufacturers’ codes do not always turn the check engine light on, but provide more information to the technician. For instance Ford sets a P1000 code when the memory is cleared. This alerts the technician that information could be missing.
Numbers in the third position suggest the type of problem shown. The following image lists the systems covered.
The final two numbers suggest the component that may have the problem. In the above example, "04" suggests a misfire on cylinder number four. This may seem like plenty of information, but dozens of things might cause a misfire of cylinder number four. Causes include, a bad spark plug, plug wire, a vacuum-leak, a bad injector, stuck, or a bent valve just to name a few.
Diagnosing the actual problem
A trained technician applies knowledge and several additional tests to isolate the cause of a DTC. For instance, code P0171 shows a lean condition. This might suggest a fuel pressure test to start. A low fuel pressure calls for voltage-drop tests of fuel pump wiring, flow tests of the system and checking the pressure regulator. A normal fuel pressure means the technician may next test air flow and possibly a smoke-test of the intake to find leaks. Professional technicians use knowledge of the system to guide them to the proper tests. No machine exists that tells us what is wrong. Proper testing and interpretation of the findings lead to the problem cause.
Making things more complex, are false codes, set by non related events. For instance a bad sensor might set a misfire code although no misfire exists. The computer can also mistakenly read a companion cylinder as cylinder four. For instance many engines use a coil to fire multiple cylinders. A problem in the first cylinder may set a false DTC on the next.
Contrary to common belief, a DTC does not tell what is wrong. It is an interpretation of data, and not information
Many other situations provide equally misleading diagnostic trouble codes. A vehicle may have a code stating exhaust-gas-recycle, or an EGR valve fault. They replace the EGR valve and the code remains. A person misled by the DTC might also replace other expensive parts in a vain attempt to fix the problem.
The actual problem is ground wires, between the fuel pump and the fuel gauge sender unit. In a previous repair, the wires are inadvertently crossed. Both are grounds and the vehicle runs. The fuel sender unit wire returns to the PCM. This causes the vehicle to set a false EGR code though EGR is fine. This type of scenario cost people a great deal and occurs all too often.
Free code check at a part store
Some part stores are eager to check codes "free." This is worth far less than nothing. With a free check, the part store sells many parts, that may not be as good as those removed, and hold no responsibility. Guess work and swaptronics are far too expensive. Worse, substandard parts create new problems that we must diagnose and repair.
The least expensive route is always with a well trained, knowledgeable professional. It’s simply much less expensive to test than to guess.