Issue with MQ6 LPG sensor

Hi Everyone ,

I start using MQ6 ,from chinesse compagny and after 3 month couple of them they stop working , What I mean by stop working , when I inject natural gas on the sensor the voltage doesnt change any more.

Usualy , the cathalyzer resistor change base on the gas consentration , and that what was before I buy them . Buy after using them for 3 month they become zero sensitive!!! knowing that the heat resistor still working!

I'm using RL load resistor 1K, it may that be the cause damaging the sensor? or sensor is bad quality?