Before you set about troubleshooting your gas patio heater, ensure it has a sufficient fuel supply and the regulator is turned on. Also, check for any gas leakages or faulty valves that may be inhibiting its supply. If all looks good, try lighting your heater with a lighter or a matchstick to see whether it starts. If your heater lights up, you are probably dealing with a faulty igniter, and replacing it will put you out of your troubleshooting misery.
However, if the ignitor isn’t at fault, you must look over your thermocouple — a safety rod connected to the valve, located between the ignitor and the pilot light. Being an outdoor fixture, patio heaters and their thermocouples accumulate a lot of dirt and dust, with many becoming homes to pests and insects. Such debris and rust build-up may inhibit the gas supply, preventing the heater from lighting. Cleaning the heater’s burner using compressed air should get it going, while the safety components can be cleaned using a straw brush or a pipe cleaner.