Have 575sci and gaffrig tach. Left engine reads 1500 at 800 rpm idle. Hooked rinda up to diag plug and it reads correct. Is this a tach problem?

Most likely a tach problem.
Swap tachs or use a jumper wire to connect the signal wire to the other tach to verify.

agi box?

575sci is not digital and does not use them.

not the tach. jumpered to other side and still reading high. Pulled the main engine plug and cleaned. No help. Does the signal come from the ECM?

Yes it does. There is another tach output lead on the engine. Its a grey wire that will have a plug in it. You can try to hook it to the wire coming from the ecm. You could also use that wire and run a temporary wire to the dash to see if it reads correctly and narrow down the problem. If the scan tool reads correctly, then the issue is not the tach driver in the Mefi and that is where the signal for the tach comes from. Must be a wire issue.
Eddie

the other grey tach wire is by the engine diagnostic plug. at 800 rpm idle the LH tach reads 1700, the RH tach reads 1500. I will run a wire from the spare tach wire on the engine to the tach