Q: Q: car wont crank or start

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hello i have already changed my fuel pump and changed my starter but now my car will not even start when i try to turn it on only the lights turn on as normal a mechanic told me it wont turn on when using the ac while driving but if the ac is kept off it works as usual he says it can be the fuse panel box but im not sure please any help the car has been in the shop for 3 weeks
My car has an automatic transmission.

Hi There, Depending on whether or not the vehicle will turn over will determine what the next step should be. If the engine does not turn over, I would recommend checking all connections related to the battery and check to be sure the starter relay is producing power to the starter. If the engine does turn over, this may be related to a spark or fuel supply problem. Things like ignition coils, crankshaft position sensor, spark plugs, spark plug wires or ignition module are all components that help to create spark and control ignition timing which may cause the motor to not produce spark or the correct timing needed to ignite the fuel in the motor. In the case of a fuel delivery problem, this may be related to low fuel pressure, faulty or dirty fuel injectors, a faulty or dirty idle air control valve (this relays information about air intake to the computer which makes fuel adjustments), or potentially a faulty fuel pressure regulator or fuel pump. Fuel delivery problems will typically not allow the proper amount of fuel to be delivered to the cylinders which results in a lean fuel condition which can cause backfiring or simply cause the motor not to start as a result of fuel starvation. In some cases too much fuel may be supplied to the motor causing too much fuel to be introduced to the motor faster than what it can burn it. This results in the motor being flooded with too much fuel also causing it not to run properly. Any of the above possibilities can create an imbalance in the proper air/fuel ratio as well which will cause the motor to run very rough or not at all. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.

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