Changed spark plugs in 2000 Durango. Alternator & check gauge light is on.

Hello, 18 is really high for vehicle voltage -- you'll want to be between 13.5 - 14.0 volts or so for normal running. The voltage regulator is supposed to maintain the proper voltage. One of two things is going on: 1) the gauge is faulty and voltage really isn't an issue. Testing the alternator and voltage regulator output is needed with a tester to get the true reading. 2) the voltage really is that high and you have a charging system issue that requires troubleshooting. Since this happened right after replacing spark plugs, I'd check to see if some kind of wiring got displaced. Could be coincidental. In some cases, the alternator can cause RF interference and disrupt engine performance, so that's important to investigate also. Was there an original cause to replacing the plugs? Or was it for just maintenance?