Non OEM thermostats have been known to cause the P0128 code. But make sure what the code is supposed to be first.

 

Type "P0128" into the search box.

 

Yes please , those aftermarket t stats are a known problem, because they don't heat up and open in the time frame the ECM is looking for.
There have been many threads about this in the past five years or so!
Also the issue of switching to green antifreeze and not staying with orange Dexcool!!

 

Well I did use an aftermarket thermostat but I stayed with The orange stuff.

 

K then just get the correct t stat.

http://www.jcwhitney.com/p3042205/sku-3503285.jcwx

 

Thanks Oldblue. I did replace the alternator on Monday and the code went away. I ran the for 20 minutes on a test drive with my scanner plugged in and the code / battery light didn't come back on. However I will get a new OEM thermostat and replace the aftermarket one I put in last year. I think it was a half hour job at most.

Thanks for the help everyone.

 

Great problem almost solved!