Highly doubt all probes and temp sensor all fail or go out of calibration at the same time....in fact, there's virtually no way. Disconnect ALL inputs and sensors to the main unit and reconnect ONE AT A TIME. It's very likely you have a shorted temp probe and its pulling the voltage down in the control and giving you garbage data. If anyone thinks 4 probes can fail at the same time, please let me know what "broken" equipment you might have for sale.....hahaha!
I love my Apex units, never used anything different but the GHL sure has a lot of fans and they can't all be wrong.
If you can't get anywhere with the diagnosis, I'm in Foxboro and happy to connect your probes to my test equipment and help you out. Take the time to look for a pinched or mashed wire before you jump in the car as I'm willing to bet you have a short in one of them. Might be physical damage to cord....
Hook them up one a time and let us know how it behaves. Best of luck!