Q: 1995 Riviera 3800 Series II check engine light on and won't shift into O.D. at all but engine runs colder than usual.

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Buick series II non-supercharged, revving @ 1100 to1400 RPM one morning. Back to normal the next day, but check engine light is on and car will not shift into O.D. Smooth running and shifting. Very well cared for and seldom, if ever, goes over 2500 RPM under any circumstances. Trans fluid changed every 25k miles. Fluid red and normal. Temp gauge runs cold most of the time now. I did change to 180 degree thermostat while changing the water pump months ago, but was steady in all temps until three days ago. Hard to find diagnostics on 1995 model.

My car has 180000 miles.
My car has an automatic transmission.

If the engine does not reach operating temperature in a certain time the computer will go into open loop and may even lock out the overdrive if the problem continues. You will need to check engine codes to do pinpoint test on the codes stored and if thermostat is stuck open then it may need replaced or you may have a coolant sensor failure.

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