These are some of the common things your mechanic should check
Starting a diesel engine during cold weather can be frustrating if the engine is not properly maintained for lower temperatures. Batteries that are weak may not crank the starter motor fast enough or long enough to start a cold engine. As the temperature goes down, so does battery capacity. The engine will be 2.5 times harder to start at 0 degrees due to thicker oil and resistance to movement of internal moving parts. Starting a diesel engine depends upon three things; the flow of diesel fuel, heat at the point of combustion, and a battery with enough cranking power to turn over the engine. All these three elements are tested by cold weather.
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