my engine misfires only when temperature outside is under 40 degrees. on 2002 Nissan Frontier

it does not matter how long you drive as long as outside temperature is low it misfires. bring it into the warm shop and it will start running fine. It also triggers a check engine light to come on.

Asked by for the 2002 Nissan Frontier
You need to retrieve the trouble codes NOW. Do not drive truck untill you get code/codes. Otherwise you may have the expense of catalytic converters. That is much less expensive than going to a parts store to get the code/codes free.
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the check engine light is from the missfire. you need to check the plugs, wires, injectors, coolant sensor and check for vacuum leaks.