E04 Open limit (main limit, rollout or reverse airflow)

What the control is telling you

On S8B1/S8X1/S8X2 one code covers any open high-temperature safety in the HLO-HLI loop: main thermal limit, flame rollout switch or reverse airflow switch. If any limit is open, PS1/PS2 see no 24 V.

Applies to: Trane S8B1 / S8X1 / S8X2

Likely causes

• Dirty filter or blower wheel, closed registers, high static pressure
• Over-fire (high manifold pressure)
• Blower not running or stopping early
• Flame rollout from blocked heat exchanger/vent
• Failed limit

Test and repair steps

1. Warning: Never leave a limit switch jumpered; jumpers are for brief testing only.
2. Find which switch is open measure 24 VAC across each switch in the limit string during a heat call.
3. Main limit open check filter, blower wheel, return size, static pressure versus nameplate and the temperature rise.
4. Rollout open check vent, heat exchanger and burner flame pattern before pressing the red reset.
5. Reverse airflow open confirm the blower keeps running through the whole heat cycle and the motor harness is tight.
6. Replace a switch only if it is open below its rated temperature.

Settings for these units

Return filter size by cabinet width · Trane S8B1, Trane S8X1, Trane S8X2

Filters are field supplied and mounted outside the cabinet.

Over 1600 CFM upflow: return on both sides, one side plus bottom, or bottom only.

Models that show this code

25 models in Trane S8B1, Trane S8X1, Trane S8X2

Other codes on the same units

• E2.2 Flame recycles exceeded - flame lost after lighting (1 hour lockout)
• E2.3 1st stage gas valve not powered when it should be (10 times)
• E3.1 1st stage pressure switch (PS1) closed when it should be open
• E3.2 1st stage pressure switch (PS1) open when it should be closed
• E3.3 2nd stage pressure switch (PS2) closed when it should be open
• E3.4 2nd stage pressure switch (PS2) open when it should be closed
• E05 Flame sensed when no flame should be present
• E6.1 120 V polarity reversed