I have a Scan Gauge II plugged into my car at all times to monitor MPG. Last week the battery was replaced, and I went through the procedure to "ready" the car (drive 1800-2200 rpm for 6 min, stop in D for 70 sec, drive 1800-2200 for 5 min, stop in D for 1 min, drive 1800-2200 rpm for 4 min, stop in D for 1 min and shut off). I did this twice, and as I was driving the scan gauge finally said "Ready, no codes". I was careful not to hit "clear" and just shut off. This morning I have "Not ready" again. What's up? How was it ready last night?