Q: 05 Buick rendezvous won't start new battery new starter and alternator is good what could it be

You have not specified whether the starter motor works or not versus a circumstance where the engine does crank (turn over) but won’t catch and run. I will cover both scenarios for you. Check to see if there are any stored diagnostic trouble codes. Be sure you have a fully charged battery that passes a load test. Confirm that the engine immobilizer system (security system) is not activated thus preventing the car from starting. If the immobilizer system is on, you may see a security warning light. If the warning light is on, to temporarily override the immobilizer system, see these instructions. If your circumstance is no starter motor operation at all with the key held in the "start" position, the procedure is to test for power and voltage drops to the starter motor/solenoid. If there is no power, or a large voltage drop is measured, then the electrical circuit supplying the starter motor has a fault. The circuit that will have to be tested begins at the battery and includes grounds, wiring (some of which is very heavy cabling), fuses, relays, the ignition switch, neutral start switch, and terminations. Basically, with a fully charged battery, and with the key held in the "start" position, the starter is either getting power or it isn’t. With the key in the "start" position, if the starter motor is getting power but the starter doesn’t work, then the starter is condemned and replaced. Note that starters can and sometimes do develop intermittent faults due to bad spots on the armature. If you want the foregoing diagnostic steps performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a no start diagnostic and the responding certified mechanic will get this taken care of for you.

In the other scenario, if the starter motor does work and the engine cranks at adequate RPM but just doesn’t run after a short period of cranking, be sure there is adequate fuel in the fuel tank and that the fuel is not old. Download any OBD-II trouble codes that have been stored and examine those codes for diagnostic clues. Check for adequate fuel pressure, injector operation, and a spark at the spark plugs. Basically, in this scenario, if the starter motor is turning the engine over rapidly, at sufficient RPM, but the engine does not catch and run, that means that there is an ignition, fuel, air induction, or mechanical fault in the engine that will have to be repaired. Check for vacuum leaks, an air induction fault like a stuck idle air control valve or, considering fuel, check for a failed temperature sensor that must be working properly to signal the PCM to enrich the mixture on cold start. Other possible no start faults are low compression, defective spark plugs, a defective coil(s), a failed crankshaft or camshaft position sensor, a failed throttle position sensor, and a broken timing belt. Regardless of the underlying cause, if you request a no start diagnostic the responding certified mechanic will get the problem diagnosed and repaired for you promptly, right at your location. Please let us know if you have further concerns or questions as we are always here to help you.