DKN AM-E exercise bike: Fails to wake on button press - requires power cycle

[mahi]

A friend has a DKN Technology AM-E exercise bike (hometrainer) that developed a strange issue: When it has not been used for a while (anywhere between hours to days), it no longer wakes by pressing buttons or pedaling. Simply unplugging power for a brief moment fixes the issue, but then the previously used program and settings are lost.

I took a look at the board, but did not see obvious signs of damage. I removed and checked capacity and ESR of the 6 electrolytic capacitors (4x Chengx 100µF/25V, 1x Chengx 220µF/25V and 1x Fengle bipolar 100µF/25V). All were reasonable. Nevertheless, I replaced the unipolar ones with new Rubicon capacitors. That did not fix the issue.

All buttons and the pedal sensor are connected directly to the Fujitsu MB95F378E microcontroller with external pull-up resistors and ground the microcontroller input when pressed/activated.

When the bike does not receive input for a couple of minutes, it goes to sleep. Pedaling or pressing any button wakes the unit. If you put a microcontroller to sleep, you can wake it with interrupts. However, apart from the pedal sensor, none of the buttons are connected to an interrupt pin. So, I'm not even sure the microcontroller enters sleep mode. It may simply turn off the display.

I'm pretty sure it's not a problem with the power supplies on the board. When the problem occurs, the 3.3V power rail is present on the microcontroller. When I press one of the buttons, I see the appropriate pin on the microcontroller go from 3.3V to 0V, but there's no reaction. If however, I briefly pull the RST (reset) pin to ground, the microcontroller restarts and everything works normally again (apart from losing the previously used program and settings) - just as unplugging power.

So it seems the microcontroller "hangs". But what may cause this?

(Testing the board is a bit troublesome. Without the bike attached, it just shows error E-5 and does not respond to anything. The bike is big and heavy, so I can't bring it home and at my friend's place I have limited equipment. And even then it takes hours to days to reproduce)

[fzabkar]

If you remove the EEPROM, does this generate an error code?

I notice that the MCU has 2032 bytes of RAM. Is that where the settings are stored, or are they stored in EEPROM?

https://www.application-datasheet.com/pdf/cypress-semiconductor/mb95f378epmc1-g-sne2.pdf

[mahi]

Without the EEPROM the exercise bike does not work at all. The display remains dark.

I can read and write the EEPROM in a standalone programmer, but obviously I do not know what exactly its content represent.