Boiler displays E19 and will not start, often after wiring changes

Safety first: Turn off power before working on low-voltage circuits.

• Inspect thermostat wiring: Open the boiler's low-voltage compartment and check the wires connected to the TT (thermostat) terminals on the Utica control.
• Look for shorts: Ensure no bare wires are touching each other or the boiler chassis. Repair any damaged insulation.
• Bypass test: Temporarily remove the thermostat wires and install a small jumper between the TT terminals. Restore power. If E19 clears and the boiler starts, the issue is in the thermostat circuit or external controls.
• Check external devices: If using zone controls or outdoor reset modules, verify their wiring and operation per Utica's diagrams.
• Replace fuse if blown: Some Utica controls have a low-voltage fuse. Replace only with the specified rating.