400 Hz
If you have a engine failing power assurance the proper method is to check the accuracy of the indication whether it has a calibration requirement or not. Failure to check for instrumentation errors can lead to an early removal of a marginal engine.
GLSNightPilot
The OAT gage at the aircraft can be expected to be different than the wet bulb temperature used at the weather observers station. The are two different measurements with the most difference is that the weather observers are done in the shade and not out on a hot concrete ramp. The other is they don't use a dry air temp but use a wet bulb thermomenter.
The OAT gage should read within the allowable tolerance at the aircraft when compared to a calibrated source held next to it. This also applies to the temperature measurement of the onboard system.
PVL-70
While I am not directly familiar with the C+PA Check here is my opinion, don't forget, you get what you paid for :-)
The onboard system is certified to perform this operation, unless the flight manual specifically states that you can perform a manual check in lieu of using the onboard system then you can not use the manual check. It could also be said that by not using the onboard systems data that you are in fact stating that it is inoperable and therefore are flying around with inoperable equipment. Do you then make a log entry and does the MEL allow this to be inoperable and deferrable?
You are asking for a silver bullett answer. There is not one unless there is a specifc procedure to perform in case this situation arises. IMHO if something happens and you have an incident that is related to not making power then the burden of proof is up to you and you have one strike against you when the data is reviewed that the onboard system is showing a fail.
I don't think any agency will back you up for using the system when it only tells you good news. If you deviate from using that system then you should document why.