My old beater Ranger, which I have owned since it was new in January 1990, has recently begun showing the CEL every time I drive it. Since 1990 is way before OBDII, what are my options for diagnosing the problem? It doesn't run any differently that it did before the light showed up, except for a possible drop in gas mileage from 24-25 to 21-22.

Would the need for a simple tune-up (i.e. spark plugs) trip the light? I wouldn't think so unless there was an obvious misfire, which there is not.

I don't want to throw a whole lot of money at it, since at this point in time this is a truck whose value varies from day to day depending on how much gas is in the tank.