Troubleshooting the E-93T Transformer 24 VAC Failure/Not Connected Error Code

Overview

This error indicates that the GMU (Gas Management Unit) has detected a loss of its 24 VAC supply voltage from the transformer on the TR1 and TR2 input wires. The 24 VAC circuit powers the GMU ignition controls and gas valve control circuitry.

When 24 VAC is not present:

• Ignition operation is not possible.

• Gas valve operation is not possible.

• The unit will not light or operate normally.

Common causes include:

• Failed 24 VAC transformer.

• Open or disconnected wiring between the transformer and GMU.

• Loss of line voltage power to the transformer.

• Loose connections at the transformer, terminal block, or GMU.

Important: After the problem is repaired, a power cycle is required to clear the error.

Call Avoidance

Before dispatching a service technician, perform the following checks:

1. Have the customer power the unit OFF for 30 seconds and then restore power.

2. If the error returns, schedule service.

Field Troubleshooting

Recommended Parts

• 24 VAC transformer

• Wiring harness or field wiring

• GMU Board

Recommended Tools

• Digital multimeter

• Basic hand tools (screwdrivers, nut drivers, etc.)

Troubleshooting Steps

1. Inspect the control compartment for obvious wiring damage, loose terminals, or disconnected connectors.

   • If wiring damage is found, repair it and proceed to Step 6.

   • If no damage is found, go to Step 2.

2. Measure voltage across transformer terminals TR1 and TR2. Verify the transformer output is approximately 24 VAC.

   • If 24 VAC is present, go to Step 4.

   • If 24 VAC is not present, go to Step 3.

3. Measure the incoming line voltage supplying the transformer. Verify voltage at the transformer primary terminals according to the unit wiring diagram.

   • If line voltage is not present:

     1. Inspect the terminal block wiring in the power supply box.

     2. Check fuses, disconnects, breakers, and wiring connections at the BCB boards.

     3. Restore power to the transformer.

     4. After repairs, go to Step 6.

   • If line voltage is present but no 24 VAC output exists:

     1. Verify transformer is not tripped and, if so, try to reset. If it trips again, replace the transformer.

     2. Go to Step 6.

4. Verify continuity and secure connections.

   1. Check for continuity between:

      • Transformer terminals TR1/TR2 GMU

      • 24 VAC input terminals

      Check for secure connections:

      • Loose terminals

      • Damaged wires

      • Corrosion

      • Broken conductors

   2. Do one of the following:

      • If wiring problems are found, repair or replace the affected wiring. Go to Step 6.

      • If wiring is good, go to Step 5.

5. Measure voltage directly at the GMU 24 VAC input terminals.

   • If 24 VAC is not present at the GMU:

     1. Reinspect the wiring between the transformer and GMU.

     2. Repair the wiring and proceed to Step 6.

   • If 24 VAC is present at the GMU, replace GMU.

6. Clear the fault by:

   1. Restore all wiring and panels.

   2. Cycle power to the unit.

   3. Verify the E-93 T fault has cleared.

   4. Confirm ignition operation and gas valve operation are functioning normally.

   If the fault returns, repeat troubleshooting beginning at Step 2.

Other Possibilities

Intermittent Power Loss

Loose terminal connections, vibration, or intermittent building power may cause occasional loss of 24 VAC. Action: Tighten connections, inspect terminal blocks, and monitor voltage stability.

A shorted component on the 24 VAC circuit may overload the transformer and cause low or missing output voltage. Action: Isolate the 24 VAC circuit and inspect connected devices for shorts.