There are some preprogrammed points where you are prompted for service. These can also be reset by a Mitsubishi dealer when you have service, so they prompt again after the suggested interval. I don't know exactly what yours means because I don't know what services you've had done, or when or where. But at 91K, if you haven't had it done that's probably a prompt that it's time to replace the timing belt. You should also replace the water pump at the same time, since it's essentially the same labor and the pump is inexpensive. Mitsubishi recommends that maintenance every 60K miles, and I'd back them up - my 2004 Endeavor has 238K on it and it's still running great. Well worth the periodic cost of that repair, along with the regular oil changes.
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