More than limit switches if burner is turning on but heating is poor you should check cycling themostat and then blower motor, lint filter and parts comprised in blower assembly.
When the problem is the high limit, there is no heat at all.
When the problem is radiant sensor, there is no heat at all.
Poor heating means either: defective cycling thermostat; clogged air ducts; defective blower system, problem with timer or temperature selector.
If the heating turns off in the middle of the cycle check also door switch.
Here Cycling thermostat:
The cycling thermostat is located in the blower assembly. Here removal instructions. Disconnect power first:
The thermostat can be tested using a multimeter. Set the multimeter on impedance (Ohms). Good thermostat will read about Ohms at room and will read open (infinity) at 150F.
Once in there also check the thermal fuse and check blower assembly. Remove lint and make sure blower is working. Thermal fuse is tested reading impedance across its two contacts. Remove fuse from circuit and touch with multimeter probes fuse's contacts (same for thermostat). If fuse is good it will read zero Ohms or very little impedance. a bad fuse reads infinity.
Check also the exhaust. Make sure there is no blockage.
Operations are illustrated from page 72 of service manual: Here the service manual.
About the flame sensor, that is a part that rarely fails. If the dryer ignites the radiant sensor is not responsible for the fault.
The radiant sensor is mounted in parallel with the coil.
When temperature is reached the sensor opens up allowing power to burner valve (valve 2). Testing the flame sensor like a thermostat is difficult because it opens up at high temp. I read volts while operating. I suggest users carrying only maintenance that can be performed with power off.
If the burner shuts down at random more than the sensor check the door switch. Run a cycle with door switch contacts jumped.
Start checking cycling thermostat and air ducts, those are problems that usually cause low heating.