I have used LNAV and VNAV with no problems up till now.In my last flight though after take-off I armed VNAV and LNAV but when I selected CMD A, LNAV turned off. After some trial and error I found that I could use the CMD B autopilot in LNAV and VNAV with no problem. Any ideas what I might be doing wrong?

Check failures as your FCC A might be broken if you can't use CMD A. Make sure you don't break the AP using your joystick (read the intro, it's explained there).But I've certainly come across the issue that LNAV wouldn't engage after takeoff despite definate perfect engagement criteria.

Thanks. I have service based failures on. I have checked the failures in the CDU. How can I tell if a system or item has failed? There is no indication that anything is wrong.I read the point in the manual about breaking the autopilot by forcing the yoke. I don't think that is the issue, but that's a great level of detail on PMDGs part :-)

When you go to the failures page it would say ACTIVE FAILURES at the top right away, hard to miss. Click on that if there are any. Can be cleared altogether from there. Otherwise I don't know. I've had numerous weird AP-dont-wanna-engage issues here and there, still fiddling around.

I had exactly this problem today. I could not activate LNAV, anything else worked. I switched to Cmd B and then I could activate LNAV. I had no failures (ACTIVE: (NONE))