It sounds like you're reading diagnostic fault codes. Those never say to replace parts. They only indicate the circuit or system that needs further diagnosis. To know what to fix, first you have to diagnose what's wrong. As for the transmission control system, there is a diagnostic manual just for that one system that is over an inch thick. There are hundreds of potential problems each with their respective causes and solutions.
There's only about a dozen sensors on the engine but there's over 2000 diagnostic fault codes related to them and various operating conditions. You need to be way more specific than "knock sensor 1 circuit (bank 1)". That's where your mechanic will START with the diagnostics. He may find a sensor is defective, but it is just as likely there is a wiring problem related to the sensor listed in the fault code. Many well-intentioned do-it-yourselfers think they just have to replace the sensor in question, and the guys who will read those codes for you for free at most auto parts stores are happy to reinforce that myth because that's what they do. They sell parts. When that new part doesn't solve the problem, you have just introduced a new variable into the mix that will often confuse the Engine Computer and cause additional running problems that make finding the original one even harder.
Most scanners will display "MIL request - yes", or "on" to indicate the Engine Computer wants the Check Engine light turned on. That is required when the fault detected is related to something that could have an adverse effect on emissions. About half of the codes that can be in memory do not turn the light on.
Tuesday, December 11th, 2012 AT 8:08 AM