If you suspect a defective 'UV' scanner, it can be checked by placing a DC milliamp meter in series with the purple detector lead. Turn the pilot gas OFF and press the IGNITE PILOT push button on the control panel. The milliamp meter reading should zero or NO change. Now, turn the pilot gas ON and press the IGNITE PILOT push button. When the flame lights, the meter should read approximately 200 milliamps. Turning the pilot gas ON should cause the current reading to drop to zero.
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Source: Quality Holdings LLC (Community Member)