Alto-Shaam CT Express Combi Error Code E99: Unit Mismatch
E99 RB is in Fahrenheit and DB is in Celsius means the relay board is set to Fahrenheit while the display board is set to Celsius. Change the unit configuration in the setup menu so both boards use the same unit.
Low: a configuration mismatch that usually clears from the setup menu.
- Code
- E99
- Maker
- Alto-Shaam
- Equipment
- Combi ovens
- Applies to
- Combitherm CT Express / ESi / CCi (older 4-10 and 4.10 units; boiler or generator electric/gas)
What does Alto-Shaam error code E99 mean?
Two boards on the CT Express each hold a temperature unit: the relay board (RB), which runs the oven, and the display board (DB), which shows temperatures. They must use the same unit, so the control checks them against each other. If the relay board is in Fahrenheit and the display board in Celsius, the control posts E99 RB is in Fahrenheit and DB is in Celsius. The manual's remedy is to change the unit configuration in the setup menu. Only if the message persists does it point to hardware: a defective ribbon cable, a defective relay board connector or a defective display board connector. The manual also lists a control programming issue among the causes, which is the setting itself. E99 is the mirror image of E98, where the units are the other way around, so check which unit each board has before changing anything.
Affected models and control systems
This applies to the CT Express models 4-10ESi, 4-10ESiVH, 4-10CCi, CTX4-10E, CTX4-10EC and CTX4-10EVH, which keep a unit setting on both the relay and display boards.
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
- E99 RB is in Fahrenheit and DB is in Celsius on the display
- Message after a board was replaced or settings were reset
- Temperatures shown in a unit that differs from the settings
Possible causes
Listed in Alto-Shaam's documentation
- Control programming issue (unit setting mismatch)
- Ribbon cable defective
- Relay board connector defective
- Display board connector defective
Other likely causes — what usually sits behind error code E99 on this control. A technician confirms which one it is before any part is replaced.
- A new display board that arrived set to Celsius while the relay board kept Fahrenheit
- A partial settings change that only reached one of the two boards
Safe checks before calling for service
- Open the setup menu and find the unit configuration.
- Choose one unit, Fahrenheit or Celsius, and confirm it.
- Switch the oven off and on, and see whether E99 returns.
- If it does, write down which unit you chose and when the boards or settings were last changed.
How a technician diagnoses error code E99
- Read the code and check the unit stored on each board.
- Set both to the same unit and restart.
- If the code returns, inspect the ribbon cable between display and relay boards.
- Check the board connectors for corrosion, bent pins or looseness.
- Replace the cable or a board only after the settings agree.
- A mismatch that survives the setup menu change points to the ribbon cable or connectors behind the panel.
- Board connectors sit next to live voltage.
- Replaced boards must be set up for the oven.
How error code E99 differs from similar codes
E98 is the same mismatch the other way round, with the relay board in Celsius and the display board in Fahrenheit.
Technical reference
- Maker documentation
- Alto-Shaam owner support
- Edition
- MN-36008 (Rev. 9) 03/16
- Models / control
- Combitherm CT Express / ESi / CCi (older 4-10 and 4.10 units; boiler or generator electric/gas)
- Section
- Section C, Error Codes, pp. C-3 to C-8
- 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 Alto-Shaam combi oven error codes
Which unit is right?
Whichever you prefer, as long as both boards match.
Is E99 dangerous?
No, it is a configuration message.
Why does it come back?
A loose ribbon cable can make a board lose its setting.
Need help with Alto-Shaam error code E99?
A technician confirms the cause on site and gives you the price in writing before any work starts.