Why is it difficult to start your car when it is cold?

If the car cranks but will not start except in a cold only start situation (consistent), the condition could be caused by a failure in the fuel injection system, most FI equipped cars have a separate cold start circuit allowing the engine to start using the fuel supplied during this circuits operation which can last between 1.5 through 5 minuets depending upon make model and temperature both engine and ambient. This type of failure could be caused by the ECM (electronic control module) or a bad temp sensor, it could not be caused by a clogged fuel filter because this type of system generally shares the same supply system (i.e.: fuel pump, filters and lines.)

If the car cranks and starts cold and then after driving and stopping the engine will not crank (intermittent or constant) you may have a heat soak condition which will require the replacement of the starter / solenoid assembly. This is generally cause by armature windings and the magnets that surround the binding due to bearing ware and heat and is not caused by the solenoid.

Solenoid replacement tip - Most cars after 1970 except for the erroneous Ford or Chrysler product do not have a separate solenoid - though in most cases the separate service of a solenoid is possible by the time you remove a starter that has the same or more run time as the solenoid why would you?

Generally a battery failure becomes more evident during a cold start so the likelihood of the condition being caused by a battery is slim.

More Information:

Check the solenoid and the starter. The battery charges itself through the alternator while you're driving. If the battery were the culprit you wouldn't be able to start the car no matter how long you waited until you had it jump started. If the radio, headlights, windshield wipers etc work but the engine doesn't even try to turn over (silence when you turn the key) it's probably the starter or solenoid. the solenoid uses low current from the ignition switch (keys) to complete a higher amperage circuit for the starter. If it isn't the starter or solenoid check for loose or burned wires from the battery to the starter, and check all fuses and fuesable links in the starting system. A good wiring diagram can be found in the proper chiltens repair manual for your vehicle.