couple issues

hi everyone, i need to start off by saying thanks for the ton of info here. I've searched and searched but can't seem to find an answer to my problems i'm having. i have three issues w/ my 98 trooper. the first one, i've figured out and will take care of this weekend. P0401, which is the egr issue. thats no biggie. the next one is P0502 which pertains to the VSS, but i've never had a single problem with my speed sensor. i need to get my emissions test done so i need to have this taken care of. lastly, any time, i use my power windows, all the lights dim a little low. whether it be the headlights or the interior lights or dashboard lights. what gives, any ideas anyone. again, thanks for all the help.

Your alternator is rated for 75 amps, which is typical for Isuzu. The alternators on most comparable vehicles is closer to 100 amps, result being that when larger draws are on, noticeable dimming can occur. Isuzu also steps down the alt output on some models temporarily when accelerating. It may be worth your while to see what the alt is charging at Volt and amp, as Isuzus are known for curious electrical problems especially when the alternators are not working right. One guys opinion.
On the egr, typically it is dirty and blocked ports and tubes, followed by valve itself that sets the code.
On the VSS, here is some info, but if you are not seeing any other problems, and the CEL is not on constantly because of it, then you could try clearing the codes and see if it resets.
The vehicle speed sensor has a magnet rotated by the transmission output shaft. Attached to the sensor is a hall effect circuit the interacts with the magnetic field treated by the rotating magnet. A 12-volt operating supply for the speed sensor hall circuit is supplied from the meter fuse. The VSS pulses to ground the 9-volt signal sent from the powertrain control module (PCM) on the reference circuit. The PCM interprets vehicle speed by the number of pulses to ground per second on the reference circuit.
Conditions for setting the code are Engine is running.
Engine coolant temperature is above 60°C (140°F).
Engine speed is between 1800 RPM and 2500 RPM.
Throttle angle is between 10% and 40%.
Engine load is greater than 50 kPa.
Map sensor indicates greater than 50 kPa manifold pressure.
PCM detects no VSS signal for 12.5 seconds over a period of 25 seconds

A Tech 2 can also pull up the conditions which were present when the code set, The PCM will turn the MIL "OFF" on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present. A history DTC P0502 will clear after 40 consecutive warm-up cycles have occurred without a fault or you can clear it with a scan tool.
Since it seems to be running OK, you could clear the codes and see if or which reoccur.
Good luck.

whoa, talk about knowing your stuff. thanks, all tha info really helps. i'll be sure to look into all that and once its all said and done, i will have no more cel codes. thanks for your help and input.