Quote Originally Posted by EAD15 View Post
According to the service manual, the crankshaft position sensor is what sends the tach signal.

Located on left rear of engine, below oil filter (per the manual).

At $30 on amazon (part 55555806), it might be worth it just to throw the part at it first before any other troubleshooting.
As of today the tach is back to acting normally, which adds to the frustration of having an intermittent and unresolved problem, and being clueless about it. Initially I thought that it might be a sensor that is failing, because it affected nothing else. If this continues to act up I'll buy the part and install it before doing anything else. Let's hope that it's only that.

All my other cars are much older than the Aveo, so I'm not used to troubleshooting and dealing with the imposition of BS sensors that regulate every minute detail of a car. IMO, on most modern machines and appliances it's the electronic parts that are the first to fail.

Many thanks for your help!