Any time you push the Test/Reset button the alarm will test the electronics are functioning correctly. The LCD will display 3 bars followed by 888 and return to 0.
The Peak Level reading is just a memory. At one point the unit was exposed to something the caused the reading. It could have been a substance that was sprayed too close to the unit, paint fumes, or actual carbon monoxide that caused the reading. To reset the peak level, push the Peak Level button, and hold it down while you press and hold the Test/Reset button for 2 seconds, then release them at the same time. After this, monitor the unit for a few days to determine if this is a recurring problem or if it was an old reading. If you don't get a reading, then you know it could have just been a random reading and you have no other event taking place. If a reading does return, reset the peak level again and start watching it every 1-2 hours, and try to narrow down when the reading is happening and be aware of what's going on around you (heat coming on or off, cleaning etc.) to determine the source of the reading.