Montague Oven Error Code Er04: Thermostat Failed Open
Er04 on the Montague V136EO-4I convection oven means the thermostat has failed open or become disconnected. The manual says to check it for proper connection and function, replace it if necessary, and contact a service provider.
High: without a working thermostat reading the oven cannot be trusted to control temperature.
What does Montague error code Er04 mean?
The oven control relies on a thermostat to know how hot the cavity is. It reads that sensor continuously, and if the circuit looks open, as though the sensor were cut or unplugged, it cannot know the temperature. Er04 is the code for that: the manual says the thermostat has failed open or become disconnected. The code is shown on the oven control display. The manual gives one cause, an open or disconnected thermostat, and two parts to the action: check the thermostat for proper connection and function, replace it if necessary, and contact a service provider. It does not provide a resistance figure, a clearing step, or a description of what the oven does while Er04 is shown, so we do not invent one. The wording failed open or become disconnected means a technician has two quite different findings to consider, a broken sensor and a loose plug, and the first check is the plug.
Affected models and control systems
Montague Legend electric range model V136EO-4I, convection oven only. Er04 is listed under Service, Error Codes on printed p. 12.
Code numbers and their meaning can change between control generations. We match the full model and control before acting on a code.
What you may notice
- Er04 on the oven control display
- An oven that will not heat or will not hold temperature
- Heating that shuts off soon after it starts
Possible causes
Listed in Montague's documentation
- Thermostat failed open
- Thermostat disconnected
Other likely causes — what usually sits behind error code Er04 on this control. A technician confirms which one it is before any part is replaced.
- A thermostat lead pulled loose at its connector during cleaning or service
- A sensor wire with a break inside the insulation near the oven wall
- Heat damage that has opened the sensor element
Safe checks before calling for service
- Turn off the oven and let the cavity cool completely.
- Do not pull on any wires inside the oven or behind it.
- Restart and see whether Er04 appears at once or only after the oven warms.
- Note whether it appeared right after cleaning, moving the range or a service visit.
How a technician diagnoses error code Er04
- Read Er04 and confirm the oven model and when the code appears.
- With power isolated, check the thermostat connection at the plug and the board for seating.
- Measure the thermostat circuit with a meter and compare it against the specification at room temperature.
- Wiggle-test the lead along its run to find an intermittent break.
- Replace the thermostat if it tests open, and run a full bake cycle to confirm.
- The thermostat plug and board are in the control area near line voltage.
- A reading of open or shorted must be compared with the maker's values to be trusted.
- The manual directs this code to a service provider after the check.
How error code Er04 differs from similar codes
Er04 is the thermostat open or disconnected. Er05 is the opposite condition, a shorted thermostat, and both are separate from the board codes Er02 and Er03.
Technical reference
- Document
- Montague Legend Electric Range, Operation Manual, Installation/Operations
- Edition
- No revision printed (file name: Final-Submission; listed on montaguecompany.com/manuals in 2026)
- Models / control
- Legend Electric Range model V136EO-4I (convection oven only)
- Section
- Service, Error Codes, printed p. 12
- Checked
- October 2026
We use the maker's document to read the code and do not reproduce it. Always confirm the complete model and control before applying these steps.
Related Montague range error codes
Is Er04 the same as Er05?
No. Er04 means the thermostat failed open or became disconnected, while Er05 means it failed to a short circuit or pins were shorted.
Can I keep using the oven with Er04?
The control cannot read the cavity temperature correctly, so we advise leaving the oven off until it is checked.
Could this just be a loose plug?
Yes, the manual lists disconnection as a cause, and checking the connection is its first step.
Need help with Montague error code Er04?
A technician confirms the cause on site and gives you the price in writing before any work starts.