Living space with heater

Carmel (Plug-In/No Battery) Troubleshooting Bulletin

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Models: AC2030TNA / TG2030TN
Published: January 2026
Source: Williams Engineering Dept.

Error Limit Switch (During Test Fire of Fresh Install)

Root Causes

  1. Limit switch not connected.
  2. Vent presence switch not activated.
Tip: Use a DMM to check continuity in the red and yellow header plate wires to see if the switch is pressed down by the vent.

Corrective Actions

  1. Verify wire connections per manual.
  2. Verify header plate installation per manual. Vent should be pressed down until clicking noise is heard.

Furnace Calls for Heat, But No Flame (Error Fire Three: 38)

Root Causes

  1. Hot surface igniter failed (weak or no glow).
  2. Combustion fan not working.
  3. Furnace valve not opening.

Corrective Actions

  1. Replace hot surface igniter.
  2. Plug in fan. Check for damage.
    • If damaged: Replace fan. Use 4 in-lb torque on screws. Fan must sit flat against the gasket and cast iron with no room for leaks.
  3. Make sure all wires are connected. See manual for other options.

Error Limit Switch (During Test Fire of Fresh Install)

Error Limit Switch (During Test Fire of Fresh Install)

Root Causes

  1. Limit switch not connected.
  2. Vent presence switch not activated.
Tip: Use a DMM to check continuity in the red and yellow header plate wires to see if the switch is pressed down by the vent.

Corrective Actions

  1. Verify wire connections per manual.
  2. Verify header plate installation per manual. Vent should be pressed down until clicking noise is heard.

Thermostat Plugged In and Turned On, But Screen is Black

Root Causes

  1. Loose power cable or thermostat wire.
  2. Internal wire disconnected.

Corrective Actions

  1. Push wire into terminal.
  2. Open back of thermostat and reconnect wire.

Thermostat Plugged In and Turned On, But Screen Flickers to Black

Root Causes

  1. Power connection is not stable.
  2. DC power jack behind control module has a short.

Corrective Actions

  1. Make sure thermostat wire is fully pushed in.
    • Check DC power jack behind control module.
    • Make sure wall plug is not loose.
  2. Replace power plug.

Error Limit Switch (After Normal Operation)

Root Causes

  1. Front panel and header plate are not installed correctly.
  2. Vent is not seated low enough on vent presence switch - it loosens with heat.

Corrective Actions

  1. Refer to "Front Panel Installation" section in manual. See also Front Panel Fitment section below.
  2. Move header plate and vent to create better contact.

Thermostat Gives Low Battery Warning

Root Cause

Harmless glitch.

Corrective Action

Clear screen and proceed with use. If issue persists, replace thermostat.

Orifice (Brass Fixture) Broken

Root Cause

Too much torque applied to valve. Be mindful of leverage length when handling the valve connections.

Corrective Action

Replace the orifice. During fresh install:

  • Fix one wrench to the valve inlet sleeve.
  • Twist in the gas inlet as you twist the valve in the opposite direction.
  • Do not push or pull the valve during fastening.

Install Pointers

  • Header plate position: Sits 62 1/4" from the bottom of the unit's legs, even when the unit is lifted.
    • You should see about 2 1/2" clearance from the header plate to the top of the furnace.
  • Vent presence switch: Make sure it is clicked down from the vent and is fully connected.
  • Gas fixtures: DO NOT use heavy force when threading any gas fixtures to the valve. Be mindful of the added length when moving the furnace into the wall. The longer the gas fixtures, the more leverage any force will have against the brass and break it.
  • Verification: Run three cycles on the unit to verify your install.

Front Panel Fitment

Pay special attention to the front panel's alignment with the furnace body.

Good vs. Bad Installation

Bad install: Gap between panel and furnace.
Good install: No gap.
Make sure panel lip wraps around furnace from top to bottom.
Source: Williams Engineering Dept., 250 W. Laurel Street, Colton, CA 92324