I have had trouble with the speedometer for awhile. Although, for the last few weeks, it seems to have been working fine. Now, it's acting even more weird, at the same time showing reduced engine pressure. This started a couple of days ago when my truck wouldn't start due to a low battery, which could be causing the computer to go haywire. I'm thinking the battery is the cause.
Why does the computer tell me, "reduced engine power?" on 2004 Chevrolet Silverado 1500
If you think you have a battery problem you should get it replaced. Low voltage can cause computers to do wierd things.