My Tach has recently slowed down, I have a aux gauge plugged into the OBD2 port... It is very consistent and accurate....Checked by Snap On hand held. It is idling at 750 RPM and the OEM dash Tach reads about 300
I would have to believe that it is not the sensor picking up the signal or my aftermarket would be off as well. As the RPM increases to around 2000, it seems to be closer but still at least 200-250 rpm slow
Is there a way to repair the Tach to read accurately, or is it a replacement only solution?

 

Do a search on this site and you will find lots of threads on this issue, 2006 tach issues, unfortunately it is most likely due to cold solder joints on your instrument cluster board.

 

I'll save you the searching!!

https://www.dieseltruckresource.com/...x-t311818.html

Read this and do a youtube search on how to remove the dash. You can fix this yourself for free.