When using the Extech CO10 Carbon Monoxide Meter you ideally want to have a low PPM reading. In open fresh air you should have a number close to Zero. Sometimes if the meter has reached and past the 30PPM Limit it will lock this number into memory. This is normal operation for the meter because you want to know the high limit. If you find yourself displaying a high reading when you are in a known fresh air atmosphere it is probably because it has tripped the meter's Recall Alarm Mode. What you need to do is Zero in a known "fresh air" area. To do this get to the 30 PPM MAX MODE Mode by Pressing the MODE Button and follow these steps:

Recall Alarm Mode: The meter will display the alarm level data setting ─“30ppm”.
Zero Mode: Press the SEL button for over 8 seconds to enter the manual zero adjusting mode. The zero value will blink and then the display will 0 and the meter will return to the Measure Mode.

Now, you have Zeroed your meter and should be seeing more accurate results.
I hope this helps!