Trane Furnace Code E4.0
Soft lockout
The high limit switch has opened because the furnace overheated. This is the newer control's more specific version of E04.
Manual wording: “Open high limit switch”
Documented for: S8V2 two-stage induced draft furnaces (FNR-SVX003A control revision)
Likely causes, most common first
- Open high limit switch
- Restricted airflow from a dirty filter or blocked returns
- Blower not running or running slow
What you can safely check yourself
- Check the air filter and clean or replace it if dirty
- Check that supply registers and return grilles are open and unblocked
- Make sure nothing is blocking the return air path to the furnace
When to call a technician
If the limit keeps opening with a clean filter and open vents, a technician needs to find why the furnace is overheating. Do not bypass the limit.
A Trane furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Installation, Operation, and Maintenance - Link Communicating or 24 Volt Gas-Fired 2 Stage Induced Draft Furnaces with Variable Speed Motor (FNR-SVX003A-EN) (Trane, S8V2A040M3PCB through S8V2D120M5PCB two-stage 80% induced draft furnaces (communicating or 24V)) · checked 2026-08-25
- Owner's Guide - Indoor Units: Air Handlers, Gas Furnaces (32-5064-WEB01) (Trane, Trane residential gas furnaces and air handlers) · checked 2026-08-25