I installed the OEM compressor myself a little less than a year ago and it has worked perfectly ever since. I use it frequently not only on my Defender but on my fleet of family vehicles. But suddenly this week, its psi readings are off by 10-13psi. In other words, if I set it to the recommended normal load pressure of say 46psi, it deflates down to around 33psi (as checked with a tire pressure tool and as read by the TPMS system) but the screen on the compressor says 46psi and it finishes the cycle, like its content that its at 46psi. To get to the correct tire pressure, I have to set the compressor to 59 psi in order to get about 46 psi. It has functioned perfectly until just recently. It's like it needs to be reset or re-calibrated for accurate pressure readings.

There’s a recent thread here regarding a software update that's available for the compressor but a dealer has to do the update (even though its an app you download to your mobile phone from the App Store). But since I installed it myself​​​, I’m doubtful that they’ll be willing to offer any support. Perhaps the dealer will be willing to repair the unit under warranty, if they just assume it came with the Defender, would they really know it wasn't port installed? I won't lie if they ask, but maybe they won't?

Additionally, I’m going to reach out to the parts dealer where I purchased it from to see if they can help with warranty.

 

(also to the other poster, my air pressure is correct ... ie with a gauge and with the dash cluster readings, they match what's on the compressor status updates pretty precisely. Within <1 PSI every time)

--I was combining this answer with an answer to the Compressor software update thread. But you get the gist.