Checked all operations in the heating mode, found system to be operating within factory specifications. Customer explained that it will sometime produce a limit switch code ran the unit for 20 min and this issue did not occur. Replaced furnace filter with customer supplied filter. Performed a humidifier tune up system is heating within factory specifications at this time. Temperatures were operating properly the whole time if system is going out on a limit then there is definitely a weaker limit switch problem. Gave customer several options upon repairing issue and customer is going to think about the options.