E4.0 Main high limit open

What the control is telling you

The main thermal limit opened because it sensed too much heat. Check both 1st and 2nd stage operation.

Applies to: Trane S8V2 only

Likely causes

• Dirty filter or blower wheel
• External static above nameplate, undersized ducts
• Return air above 85 F
• Manifold pressure too high
• Loose insulation or objects in the air stream
• Faulty limit

Test and repair steps

1. Warning: Never leave a limit switch jumpered; jumpers are for brief testing only.
2. On a 1st stage call, measure 24 VAC across the main limit: voltage means it is open.
3. Check external static pressure against the nameplate and correct duct problems if high.
4. Measure temperature rise and compare to the rating plate; check return air is not above 85 F.
5. Confirm manifold pressure is in range; correct if over-firing.
6. Look inside the cabinet for loose insulation or debris.
7. If all checks pass but the limit sees a temperature above its setting, fix airflow; if it opens below its setting, replace it. Keep test probes within 1/4 in. of the limit disc.

Settings for these units

Clearance to combustibles · Trane S8V2

24 in. front service access.

3 in. in front of the combustion air openings.

Return filter size by cabinet width · Trane S8V2

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

12 models in Trane S8V2

Other codes on the same units

• 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
• E4.1 Reverse airflow limit open
• E4.2 Flame rollout switch open
• E05 Flame sensed when no flame should be present
• E6.1 120 V polarity reversed