Way off ambient temperature reading

[Panormitis]

On December 2018 I got a Geeetech A10M.
It was working pretty much okay, after few months one of the Titan style extruders broke, so I replaced both of them with BMG style extruders from Triangle Labs.
However about one month ago I had hotend issues, the temperature was fluctuating frequently and I had thermal runaway errors, so I thought the hotend thermistor was dying or became loose.
I disconnected the hotend and removed it from the carriage to check it, and I put it back in. Afterwards the LCD was showing 200 degrees celsius as an ambient temperature (the heater was inactive at the time).
I thought it wasn't a big deal, I'll just replace the thermistor. But I also wanted to convert it to a direct drive extruder and add a Volcano hotend, so I waited for the parts to arrive. Now it has a Volcano style hotend with new heater cartridge and a new thermistor, but the printer insists the ambient temperature is 200 degrees celsius.
The thermistor type is correctly specified in the firmware, but it doesn't make any difference.

I also tried to disconnect the thermistor and connected a resistance box to try various resistances and check the temperature reading. At 0 ohms it displays 737 degrees. At 100 ohms it displays 337 degrees and pretty much from 1 kilo ohms and up it displays around 200 degrees.
So I don't think is the thermistor, it's probably the board. Everything else works, any ideas? Is replacing the board the only option?

[William]

Hi, does the printer's room temperature show 200?

You can try to remove the hot end cable from the motherboard,

See how much the hot-end temperature is displayed on the screen,

If it shows 200, there is a problem with the motherboard and it needs to be replaced.

If it shows -14, you may still need to check the hot end part.

I hope this can help you.

[Panormitis]

Hi William,
I tried that, it still shows 200. In the past I remember it showed briefly -14, and then 200. Why -14?

[William]

Hi, The temperature is displayed as -14 when there is no hot end cable on the motherboard.
But the temperature should be displayed as room temperature after inserting the hot end wire normally.
If it still shows high temperature, there is a problem with the motherboard.

[Panormitis]

Is there any way for the board to be repaired? Could you point out which SMD components are responsible for temperature measurements?

[William]

Hi, there may be some reasons for a short circuit on some parts of the motherboard,
Cause this to happen;
You need to replace the motherboard.

[Panormitis]

I figured out a workaround. I only need one extruder motor now, I won't be using mixing anymore and since the thermistor sensing circuit of E0 doesn't function correctly (probably the mosfet or a resistor died), I checked the schematic and the pinouts of the board and found out that E1 has its own heating and sensing circuits. So I swapped the E0 & E1 cable connections.
Then swapped some pins in pins_GT2560_V3.0.h to make E1 function as E0.

#define TEMP_0_PIN 9 // Used to be 11 Analog Input (8)
#define TEMP_1_PIN 11 // Used to be 9 Analog Input

#define HEATER_0_PIN 3 // Used to be 10
#define HEATER_1_PIN 10 // Used to be 3

#define FAN_PIN 8 // Used to be 9

I also put a new a4988 driver on E1 I had laying around, just in case.
Now the printer works fine!!
Can a dead a4988 driver on E0 cause all this trouble, by any chance?