Absolutely can't fault you for looking into it yourself first, but keep in mind we pay doctors a lot of money for their experience and training, but they just bury their mistakes. We hold mechanics to much higher standards but they charge considerably less per hour, and get an undeserved bad reputation if they don't fix the problem right the first time. In some cases they will save you money, especially when it comes to randomly replacing parts in hopes of solving a problem such as you're having now.
The only code listing I have is not specific for the transmission code so I can't say whether or not it's for the neutral safety switch. On older vehicles it prevented the starter from working unless the transmission was in neutral or park, and it could include the switch for the backup lights. Neither of those would have anything to do with emissions so it wouldn't trigger the Check Engine light or set a code. On newer vehicles with computer-controlled transmissions, they use a "range switch" to tell the computer which gear you've selected. I believe the thinking is if that range switch has a problem, the computer might not know which forward gear it's in, so it may default to first or second gear and stay there. While that won't increase the composition of bad gases in the exhaust, staying in a lower gear means the engine runs faster and burns a lot more fuel, so the AMOUNT of exhaust gases increases, and therefore the light turns on. Instead of the three wires typically used with a neutral safety switch, range switches can have more individual switches built into them along with more wires that can break or corrode, or they can be a position sensor similar in construction to a throttle position sensor, or a volume control on an older tv. They use a movable wiper contact that is susceptible to dirt and poor contact between that wiper and the part it slides against.
Normally transmission-related fault codes are stored in the Transmission Computer, not the Engine Computer. Most low-cost code readers only read engine codes, not the codes in all of the other computers. A generic code 700 in the Engine Computer indicates a more specific code is in the Transmission Computer and must be read with a scanner capable of accessing that computer. I suspect the people who read your codes used a scanner that was able to read codes for the transmission.
Operation of the knock sensor can be observed with a scanner that displays live sensor data. Many knock sensors have just a single wire and produce a small voltage when the sensing element resonates, or vibrates at a frequency that's indicative of spark knock. When that happens, the Engine Computer retards ignition timing to try to stop that knocking from occurring. Internal mechanical engine problems can also be falsely detected as knocking. That results in the ignition timing being constantly retarded along with lack of power, increased fuel consumption, and increased emissions. On many newer models, (to me anything early '90s is "newer"), the Engine Computer will initiate a self-test that causes spark knock to see if it gets the expected signal from the knock sensor. You may never know that self-test is taking place, but if no signal is received when it's expected, a related fault code will be set.
All my listing gives me for a description is "circuit malfunction". That doesn't specify whether the computer is never getting a knock signal when it expects to get one, or whether it's constantly getting a signal. Constantly getting a signal is typically caused by connecting rod knock due to worn bearings. On older vehicles that used V-belts, that could even be caused by tightening those belts too much. That put excessive upward pressure on the snout of the crankshaft and would bend it a few thousandths of an inch so it contacted the upper part of the crankshaft bearing. When a front cylinder would fire, the force pushed the snout of the crankshaft down until it hit the bottom half of the bearing. After that the belts would pull the crank back up. A double knock was the result once per two crankshaft revolutions on an inline six-cylinder engine or twice per two crankshaft revolutions on V-6 and V-8 engines. The clue to a trained mechanic was the sound of that knocking. Spark knock affects all six or eight cylinders at the same time and usually not at idle, just under load or acceleration. Crankshaft knock occurs at a slower rate but it still gets detected by the knock sensor.
Thursday, December 15th, 2011 AT 5:12 AM