That sucks. When the tech at Discount Tire tried to do mine the first time, it also failed for some reason. He was using a Bartec TPMS tool with the Mitsubishi adapter cable.
He walked around my car and read each of the 4 TPMS IDs into his tool. Then he connected the tool to the OBDII port (with the Mitsubishi cable) and tried to load the 4 sensor IDs into the TPMS computer. I wasn't watching him closely at that point, but this method didn't work. He barely knew what he was doing, so it wouldn't shock me if he wasn't paying attention to the exact instructions on the tool display that walk you through the process.
Fortunately, there's a second way to register sensors in a Mirage. Using the same tool (with the same Mitsubishi adapter connected to the OBDII port), you can tell the Mirage's TPMS computer that you are going to trigger the 4 sensors on the car instead of loading the IDs directly through the OBDII port. As you trigger each sensor, the Mirage computer will store the ID.
So the guy at Discount Tire started doing it this way. I knew he was up to something when he started letting air out of my left front tire. I asked him if he was trying to trigger the sensors. He said yes. So I grabbed my own TPMS tool and triggered the 4 sensors myself while he sat in the driver's seat with the Bartec tool still connected. Each time I triggered a sensor, he would tell me the Mirage's TPMS successfully detected it. After the 4th sensor was registered, he said the tool indicated success and I was done.
View my fuel log 2015 Mirage ES 1.2 manual: 51.7 mpg (US) ... 22.0 km/L ... 4.6 L/100 km ... 62.1 mpg (Imp)