If the alternator diodes are going, they can put an A/C voltage on the car. The PCM is , um, uncomfortable with that. It can cause gages to read incorrectly, or throw off PCM. Bad spark wires have the same effect.

My name is Richard. I was a Contouraholic. NOW: '02 Mazda B3000 Dual Sport, Black BEFORE: '99 Contour SE Sport Duratec ATX Spruce Green PIAA 510's, Foglight MOD, K&N Drop-in