Common problems

Message or behaviour	Probable cause	What to do
Not sure whether adapter, permissions and baud rate are right	–	obd-diag setup-check or Options → Check setup … (Installation)
obd-diag ports finds nothing	adapter not plugged in, Bluetooth not bound	check dmesg; Bluetooth with sudo rfcomm bind 0 <MAC>
“Permission denied” when opening the port	user not in uucp/dialout	Installation, section on permissions
info hangs or “no response from …”	wrong baud rate	try --baud 115200 or --baud 38400
UNABLE TO CONNECT, no codes, no protocol	ignition off (exactly what happened in the first test on the W177), or the switch on the FORScan adapter set to MS-CAN	ignition on, switch to HS-CAN (Adapter and connection)
info shows a low voltage although the battery is fine	the adapter measures less at pin 16 than the control unit (W177: 11.2 instead of 12.0 V)	normal; with a low ATRV the control module voltage (PID 42) counts, if readable
First start after plugging in: error at ATZ (?)	garbage byte in the adapter right after plugging in	is now repeated once; otherwise simply start again
“Battery voltage too low”	battery below 11.8 V (adapter and, if readable, control unit both measure too little)	charge the battery; live data then only queries every 5 s
“Connection to … lost”	adapter unplugged, loose contact, Bluetooth gone	reconnect; nothing was sent after that
“Control unit rejects mode … (response 7F …)”	control unit busy or conditions not correct	try again later; the fault memory is then unknown, not empty
Codes without description, “Trouble code catalog missing”	catalog not built	uv run python tools/build_dtc_db.py
“Clearing is disabled”	intended, until the test at the car	Reading and clearing trouble codes
Live data ends with an error after three rounds	adapter only reports bus errors, usually ignition off	ignition on
Freeze frame “none stored”	no stored code	normal

For anything not listed here: repeat with --trace and look at the trace (Sessions, report and trace).