If you have an ignition control (the Johnson G770, G775 or the new UT control) on a D*CG unit or just replaced a control and it“sparks just a few times and then shuts off”, there are three things you should check.
On all of the above ignition control, if voltage drops below 19 volts at the RO terminal, the control will lock out. You may get a few sparks and then nothing.
First, this situation can be caused by a faulty rollout switch. Remove the wires from the rollout and jump them together. Make sure they are removed from the rollout. If the unit functions properly, chances are the rollout is faulty. To verify this, ohm out the rollout. Since the contacts are normally closed, they should read “0” resistance (a dead short). If the ohms are not “0”, the contacts in the rollout could be worn or corroded causing a voltage drop through the switch. This will cause the ignition control to lock out in the rollout circuit. Also, check each side of the rollout to ground. Any reading indicates a bad rollout switch.
The second possible cause can occur when the building has a high pressure gas service. External to the unit should be a reducing regulator. This reduces the high pressure to normal inlet pressures so the unit can operate. If the vent is blocked or not functioning, or the regulator is defective, the regulator may not reduce the pressure to the valve. The valve tries to open against this high pressure but can not open. Because of the “load” on the valve, and the coil trying to open the valve unsuccessfully, a voltage drop occurs and the unit will try to spark a few times and then lock out due to reduced voltage at the ignition control. If the contractor shuts off the external gas cock and breaks the union and then turns the unit on, the valve will open and the spark will continue because the “load” has been removed on the valve. Either the vent has to be cleared or replaced, or the regulator has to be replaced to correct this situation. Be sure to monitor the inlet gas pressure through a few cycles to make sure the external regulator is not “passing” higher pressure.
Third possible cause could be a bad gas valve. Remove the PV & MV wires from the ignition control; recycle the power to the ignition control by removing and reinstalling the TH wire on the ignition control and see if the unit begins to “spark”. If it does, then you could have a defective gas valve and it should be replaced.
Keep this in mind when diagnosing nuisance lockouts on D*CG units. These check should be performed first to avoid replacing good controls. All other safeties and operating controls still need to be checked and verified but this could save the tech some time.