I was annoyed that my Kaiweets DMM defaulted to Fahrenheit in the temperature measuring mode, because I don't live in the country which uses that cumbersome unit for temperature.
So my plan was to to change that. On YT I saw that there are units out there which default to Celsius, so there must be a way.
Luckily, these DMMs store there configuration and calibration data in a separate EEPROM (24C02), similar to that with the DTM0660 chip. It is possible to read and write this EEPROM with simple tools like (CH341B).
My research indicated that the HABOTEST units seem to default to Celsius, so I got one and compared the dumps of the EEPROMS.


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The calibration data and some of the configuration seems to be equal to that of the DTM0660[1]. However, there is a lot stuff different to what is in the datasheet[2] of the DTM0660. So I did some trail and error, and finally found the bit that does the magic.
It is the 3rd bit at 0xFD: 0 means Celsius and 1 means default to Fahrenheit. E.i you have to change 0xe6 to 0xc6 at address 0xFD.
I also changed the order of the modes in the resistance/continuity/diode setting, because I need the continuity mode more often than resistance, I swapped them: Swap 0x07@0x85 with 0x09@0x95.
These hacks work at least for the two units I have. The HABOTEST has PCB revision HT118A-A1 and a blob as main chip and the Kaiweets has revision HT118-A2 and a main chip with marking DMM9803. My guess is that this hack will also work on other PCB/chip versions.
If somebody wants to do the hack these are the steps:
1. Put main chip in reset state, so that we can access the I2C bus to the EEPROM. That is done by pulling pin 55 of the main chip to ground. This pin is exposed by the unpopulated header. I used a jumper wire for that, see pic.
2. Set the selector to volts or some other mode. In the OFF state VCC and GND are shorted.
3. Read the chip with a programmer like [3] (found on ebay/aliexpres...). On linux you can use [4] as software for that. If the DMM boots on connecting the power, you will just read 0xff. In this case the jumper wire is not connected correctly.
4. IMPORTANT: Back up original dump. It contains the calibration data, and tools to redo the calibration are not so cheap...
5. Modify bytes and write back to EEPROM.
6. Be happy.

I also own a HT206D which suffers from the same illness. Unfortunately, while the EEPROM is as easy accessible like with the HT118A, main chip is different (no markings). It is also no DTM0660 or DM1106EN but similar. If some body has a version which defaults to Celsius, I would be very happy, to get a dump for comparison. Perhaps, we could start a collection of dumps of these units to revers engineer more of the configuration options. Best would be to get a datasheet, though.
[1]https://devydd.github.io/UT210E-EEPROM-Editor/
[2]https://www.eevblog.com/forum/testgear/an8008-us--10000count-1uv-0-01ua-0-01ohm-resolution-meter/?action=dlattach;attach=333782
[3]https://www.eevblog.com/forum/repair/ch341a-serial-memory-programmer-power-supply-fix/
[4]https://github.com/command-tab/ch341eeprom