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Q: 1999 Nissan Altima won't stay running. I changed the thermostat.

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I got a new thermostat today and my car won't stay running I can't go past twenty miles per hour and when I put it in gear it shuts off. please tell me it's a minor problem please help me

My car has 267856 miles.
My car has an automatic transmission.

A: First of all, it isn’t possible to tell you...

First of all, it isn’t possible to tell you exactly what is wrong with your car without direct testing, in other words, diagnosing it. If the thermostat made the car run at the proper temperature, then that is not likely the problem. If it is still running hot, then you still need to figure that out. The stalling could be a blown head gasket at the worst. Aside from that, there isn’t much else in an overheating, other than getting too hot that would cause this problem.

As far as minor, most likely it is something such as the MAF (mass air-flow sensor). If this started after you changed the thermostat, it is very common among even the most experienced technician to leave a connector loose. I would double check your work to start. It is very common for the air boot between the MAF and throttle body to be left unsealed. When it is, many strange symptoms will occur. MAF’s can be expensive, but it is much less than replacing a head gasket.

What needs to happen is a scanner needs to be connected and any codes should be retrieved, as well as all the scan data should be studied for clues. I could list a dozen possibilities, but it is not very effective testing these items without a scanner. Codes are the most effective pieces of information in the diagnostic process.

If you need assistance with these checks, consider YourMechanic, as one of our mobile technicians can come to your location and properly diagnose your stalling issue on sight.

Good luck!

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