You have made all the right moves so far, however, the P0715 code may be hiding more than you think. On its surface, it says simply the input shaft speed sensor has failed which is quite true. However, on an electronic or electrical level, you have to check to see if the wiring for this particular sensor is in good shape.
At the device itself, it is pretty easy to see if the sensor and its connections are in good shape, you remove the sensor and look. If you see any signs of rust or corrosion — pretty common in a 21-year-old car — then it is likely that the reason for the sensor error message is that either the wire loom that is carrying the sensor wire — there are three — has failed at some point by either going short or open. Though the electrical failures are different, their results are the same, the sensor stops working and merely replacing it won’t gain you much.
No, instead you will have to do what electronics techs call “ohmming out” the circuit by finding the three lines and checking their continuity from beginning to end. This is a rather difficult and time-consumming task as it requires placing a volt-ohmmeter in resistor-check mode and then moving along the wire touching test points (you will need either a schematic of this circuit or the service manual) to see if there is continuity. If there is you have to move to the next test point and so on until you have finished. In all honesty, this labor-intensive work will take eight or more hours for only the three wires. If the loom is exceptionally tight, then it could take longer as you will have to either take off the zip-ties enclosing the loom or try to wiggle the wire free to start the process. This is best left to a technician.
However, I think there is a lot more going on here. When you receive the P0715 code, though it means that there is a input shaft sensor failure it really means something else could be happening. Indeed, that’s the way of the whole diagnostic testing business, though it is getting better. You see, when your code reader finds a code and it says there is, using your own example, an input sensor failure, it also hides the fact that it could mean there are other errors, not just the speed sensor. The sensor failure code gives you or your technician a general area where to start and then you move on to either looking at technical service bulletins next, just to make sure there are none outstanding or whether there is a service bulletin on your specific error and car and then onto other possible failures related to the error code.
Since your transmission has failed completely, I think that torque converter has failed. This is one of the potential errors that the P0715 error covers. If the torque converter has failed, you can either try to replace it yourself — if you feel comfortable doing it and have the tools — hydraulic floor jack, transmission cradle lift and the like — then go for it. Personally, I think you should let a pro handle this because it is what they do every day. Their experience means that they will do the job in the fastest possible way and make the best recommendations in the first place.
By the way, the torque converter failure means the transmission won’t run at all and it your Honda will act as you describe it.
The error code hide much more than that, though, it could also mean that the reluctor ring on the input shaft has gone or it could also mean a programming glitch with your Honda’s engine control module or transmission control module.
Personally, though, I think that given the age of your Honda and the results after you swapped out the fluid and sensor, it’s more than likely that it is the torque converter. At least it is a specific device that can be changed which means you don’t have to change the entire transmission, but, given the fact that the converter, if it is replaced, will be working at peak efficiency, then it is possible that other time-weakened devices such as the impeller or the planetary gearing or even the clutches and bands or other parts, will go bad, leaving you with further costly problems — the torque converter repair is roughtly $1,400 with time and labor and if the other parts begin failing you are looking at equal costs.
It would make sense at this point to think of dropping in a rebuilt transmission which will cost you about $3,450. I hope this helps. Let me know the outcome.
