component failures in any system will "take it down." Even in simple systems, the troubleshooter must always consider the possibility that there is more than one fault. (Replacing each component, using serial substitution, and then swapping each new component back out for the old one when the symptom is found to persist, can fail to resolve such cases. More importantly, the replacement of any component with a defective one can actually increase the number of problems rather than eliminating them). Note that, while we talk about "replacing components" the resolution of many problems involves adjustments or tuning rather than "replacement." For example, intermittent breaks in conductors --- or "dirty or loose contacts" might simply need to be cleaned and/or tightened. All discussion of "replacement" should be taken to mean "replacement or adjustment or other modification." See also 5 Whys Bathtub curve Cause and effect Debugging Forensic engineering No Trouble Found Problem solving Root cause analysis RPR Problem Diagnosis References Problem solving