IMPORTANT: Troubleshooting of heaters should only be carried out by qualified Gas Safe engineers or Electricians.

Red light permanently on - The heater is in an overheat lockout or permanent lockout. It can be reset using the reset button. If the reset does not work, turn off the mains power supply for 10 seconds, then turn it back on again. 

For older models like the CTU and CTCU, press the red button if the heater  has sa dual-button fan limit stat or the red button on the Therm-O-Disc inside teh heater. Check the inlset pressure on the Gas Valve. If the heater still locks out, Contact Technical Support for further troubleshooting.