Q: Intermittent "no start" or dies while running fine on highway - what's wrong? 2003 Buick Rendezvous

Hi there:

Once the vehicle starts, the job of the battery is pretty much done; which means if you’re having issues with the vehicle shutting off, it’s most likely caused by ignition system components; such as a the distributor or the crankshaft position sensor perhaps. It’s possible that you have battery cables that are corroded or a battery that’s losing cold cranking amps. Since there are multiple items that could be causing your issues, it’s probably a good idea to have a professional mechanic complete an engine is hard to start inspection.

Hello - you don’t indicate if either the "no start" or "dies while running down the highway" scenarios results in a Check Engine light condition, but I would check for problem codes either way. For both stalling and no-start conditions, a failing crankshaft position sensor or a failing fuel pump/faulty fuel pump relay would behave similar to what you are experiencing. I believe these problems have the same source. I would recommend a thorough inspection of the fuel supply and ignition systems completed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.