No spark at the plug

Possible causes:

• The Ignition device (transformer) is not grounded. Clean the point where the box connects to the frame of the heater and re-attach. Check for continuity.
• The ignition device has power going in- no spark coming out. Since you have already determined that the box is grounded, then the ignition device is defective- replace.
• There is no power into the Ignition Device but there is power into the Safety Control (reset). The Safety Control either has a bad connection or is defective- repair or replace.