1996 GMC Jimmy transmission problem, NO OVERDRIVE

redtaileddoveredtaileddove Member Posts: 2
edited May 2015 in GMC
I have a 1996 gmc jimmy with aprox 150000 miles on it. I noticed it wasn't shifting into overdrive and the check engine light came on throwing a p1870 code. My mechanic changed the trans filter and sent me on my way.....still no overdrive. He suggested maybe a speed sensor......I would prefer not to guess at this point. The truck shifts flawlessly in all gears, just no overdrive.....anyone have any suggestions?
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