I have a strange problem with 2004 Chevrolet Venture'* stock CD + radio. CD-* play fine, I can tune to stations in AM and static (including voice if frequency near a station) can be heard on FM when not tuned exactly to a station. However, when I tune to a station in FM, the sound becomes really quiet (can hear it quietly when volume turned to max). The radio also displays a calibration error message when started and I don't know if the stereo has ever been changed.

Where should I start looking for the error? Finding somebody with a Tech2 and dealing with the calibration error? Testing the antenna connection? Somewhere else?

https://charm.li/Chevy%20Truck/2004/...udio%20System/

I would check the antenna connection to the radio, then check the antenna itself for damage. If both are fine and the problem continues, I would trace the antenna cable to see if it is damaged anywhere.

Welcome aboard!

Checked the antenna connections - everything seems to be okay there. Also, when I remove the antenna, I can hear only static and stations cannot be found; if attached, it finds stations but they are quiet.
What could it be? An issue with the auto-volume feature? (would probably affect playing CD-* as well, wouldn't it?) Maybe a different spec radio than the original? I would just change the radio but then it would also need to be reprogrammed and I don't have a tech2.

I'm thinking the receive amplifier has gone bad. Not something fixable except by a shop that can diagnose and solder circuit boards . . . which there are almost none of any more. A good test would be to swap the head unit, but like you said, it would have to be re-VINned.

You have a bad audio amplifier. Replacement or repair is necessary.

I was thinking that too, except that CDs play fine on it. That pushed me towards the receive-amp circuit. I thought that both radio and CD pass through the audio amp. Not sure, because these radio heads just about never do stuff like this, but seems likely.

The receiver has it'* own set of amps internal.