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So, is it going into 4th without lockup? If so, RPMs will only be about 200 higher than normal, which will hardly affect your gas mileage. Or are you saying it won't even go into 4th?
Since neither 4th nor lockup will engage if the coolant or the trans fluid is really cold, it could just be the weather if you're in an extreme climate. Or a sensor could be bad, or its electrical connection. On my truck, 4th will not engage until the coolant reaches 60F, and lockup won't engage until the coolant reaches 140F. The latter can take 3-4 miles of driving in winter. And occasionally if the truck has sat for a month, it still won't engage for several miles after that, until I shut down and restart.
Since neither 4th nor lockup will engage if the coolant or the trans fluid is really cold, it could just be the weather if you're in an extreme climate. Or a sensor could be bad, or its electrical connection. On my truck, 4th will not engage until the coolant reaches 60F, and lockup won't engage until the coolant reaches 140F. The latter can take 3-4 miles of driving in winter. And occasionally if the truck has sat for a month, it still won't engage for several miles after that, until I shut down and restart.
