Does the "Maintenance Indicator" sign on a METAR imply we can't trust it?
Not necessarily.

Automated stations have a number of sensors for detecting,


Wind speed and direction
Visibility
Obscurations to vision
Cloud coverage and ceiling
Temperature and dew point
Barometric pressure and altimeter setting
Precipitation
Freezing rain (Icing)
Lightning (thunderstorms)


It is likely that some of these are not functioning properly, while the others are working correctly. In such cases, the METAR reports contain sensor status indicators which give information about the sensor which has failed. For example, consider the following METAR:


  KMEM 230853Z AUTO 18014G18KT 10SM CLR 16/M02 A3008 RMK AO2 SLP117
       T01561022 TSNO $   


Here, TSNO indicates that the automated station is equipped with a lightning detection system and that sensor is not operating. Following is a list of sensor status indicators:


RVRNO- Runway Visual Range is missing.
PWINO- Present weather identifier sensor (is fitted and) is not operating
PNO- Tipping bucket rain gauge sensor is not operating.
FZRANO- Freezing rain sensor is not operating
TSNO- Lightning detection system sensor is not operating
VISNO_LOC- Secondary visibility sensor is not operating.
CHINO_LOC- Secondary ceiling height indicator and that sensor is not operating.