My 2006 sonata sometimes starts up in 3rd gear and refuses to shift or while driving no matter the speed it will shift out of what ever gear into 3rd. Isn't a transmission issue as the transmission was changed 100,000 km ago, seems to me a computer issue. Have taken it into the shop several times to have codes read, first was saying ecm power supply so I cleaned the connections and it worked for a day then everything happened again. Had codes read again and it said low voltage. I cannot for the life of me find the source of my issues, everytime I drop it off at the mechanic it works great for them. I don't know the codes as I don't have a reader, trying to get my hands on one

 

Do you know how to use a multimeter? Maybe we can offer some guidance if you manage to get hold of one.
A lot of gents today and that includes some service personnel are not very confident on how to check some systems with a meter.
Also, codes are usually designed to notify when a system is not up to par, or is receiving inaccurate input info into the computer. Codes don't always tell that a particular component is defective. The codes usually provide some direction to pursue.It could be the component, the info coming into a component or even the info a component is sending to the computer. Most computer circuits in automobiles operate on very low voltage signals (milliamps /millivolts) .001 volts. The slightest resistance from a poor connection can alter the value of the signal received by the computer and the computer could be designed to provide a certain response to an inaccurate signal or information.
So,,,,just thinking you have a code is no guarantee the cure is at hand. Wiring problems are very common as are poor electrical connections. What is often required is a schematic wiring diagam to examine what is on the circuit . For example , the computer may provide an operating voltage supply to several components on a string/circuit.
Any one of the components on that circuit can be shorted , for example, and pull the voltage down for everything on that circuit. Now he code will usually provide the circuit to trace down. The rest is up to the tech. If he/she is good at chasing down problems ,sometimes the problem can be found quickly. Other times it can take much longer and intermittents can take ,A LOT longer to locate. Hopefully this long winded explanation will help to understand why some problems take time to find.