Boiler water not getting as hot as it used too

I have an old boiler that I installed a Honeywell AQ475A outdoor air reset control on 10/30/05. It's worked fine up to this winter. The water doesn't seem to get near as hot as it used to when it gets down to 10 degF At that outdoor temp, the boiler water should get to around 160F, but this year it only gets to 140 at best. Has the outdoor sensor gone bad? Is there a way to check it? Is there a resistance curve available for the sensor so I can tell if the sensor is bad by using a ohmmeter? The spec sheet states that the sensors should be 10K ohms at 77F, but what should the resistance be at 20F, or 10F? The AQ475A goes through it's test sequence with no problem and no flashing LED's afterwards, but that only checks if the sensors are shorted or open. The boiler hi temp is set at 190F, the low at 120F and the outdoor low is set at -20F The system has worked fine until this winter. Any help will be greatly appreciated. Thanks- Henry Z