Calibrated NH3 gas-sensor (SEN0469) are not calibrated

Hi..

I have bought five NH3 calibrated gas sensors (SEN0469). I am using I2C i/f and DFRobot_MultiGasSensor v2.0.0 library.

I have used the same setup with other gas-calibrated modules (i.e. for CO & O2) and worked fine.
 

But I have problems with these NH3 modules (i've tried 2 out of 5). When reading their type (with queryGasType) i get nothing (respective AllData[4] byte is 0, while it should be 0x02 for NH3) and also when i read the calculated conentration values (AllData[2] & AllData[3]) they are always 0, even under high NH3 concentrations. In the meantime, when in parallel i read the respective analog voltage of the sensor using  the getSensorVoltage() function, i get flactuating values (from 58 to 280) following the NH3 concentrations.

To me, it seems that the respective modules are actually uncalibrated :-( … 

Any hints/suggestions are highly appreciated..

BR,

Kostas