I have Payne 90+ 350MAV unit comes on burners ignite + stay lit but won't push much heat out. Changed filter no codes

If the burners are at full gas and blower is running, check for a blocked plenum chase or return. Good way to check is to turn the heat off, let it cool until fan is off and then turn the fan ON using the manual thermostat control. What happens?

If the burners are at full gas and blower is running, check for a blocked plenum chase or return. Good way to check is to turn the heat off, let it cool until fan is off and then turn the fan ON using the manual thermostat control. What happens?

The fan comes on

the fan comes on

Does it blow at full speed, lots of air?