Checking the Basics

Diagnosing problems in modern automobiles can often be very frustrating and confusing, especially when it involves computer controlled systems. The truth is, automotive electronics are proven to be quite reliable and most problems with new cars are the same kinds of problems that older cars without computer controls have. These include worn mechanical parts, fluid leaks, overheating, oil burning, vacuum leaks, bad spark plug wires and/or spark plugs, dirty filters, restricted mufflers and exhaust systems, exhaust leaks, and all the other familiar problems.
Don't overlook the obvious. Before suspecting a computer problem, always check under the hood for all the same things you would look for on a non-computer controlled engine. If the engine "feels" like it's running out of fuel, check the fuel filter(s), pump, and hoses. If it feels like the timing is too far retarded (poor response - lack of power), or too far advanced ("pinging"), check the ignition timing. Do these kinds of checks even if the computer's self diagnostics have stored trouble codes. Sometimes a mechanical or tune-up related condition, as simple as a dirty air filter, will cause sensor signals to deviate far enough from the norm to set a trouble code, or even a string of codes.
When a fault is suspected in a computer controlled circuit (indicated by a code), check the battery and connections, make sure the battery is fully charged and the charging system is operating properly. Check all plug-in connectors for damage or corrosion. Make sure connections are clean and tight. Check for ground wires that may be broken off at their terminals, or left off by another mechanic who performed some mechanical repair and overlooked them (yes it does happen). Look for wires that may have the insulation worn off from rubbing against sharp or rough metal edges, or melted off by touching an exhaust system part. Look for signs of tampering, such as jumper wires bypassing resistors or switches, or add on electrical accessories improperly connected. Make sure that spark plug wires are not too close to sensitive components (such as oxygen sensors, knock sensors, etc.) or their wire leads. Be thorough! You may save yourself a lot of time and avoid chasing after "ghosts".