If you think it's low voltage, the first step would be to actually ascertain the voltage at the battery. The three light meters are very imprecise and, if only for that reason, of little accuracy. If the voltage at the battery is reasonable, I would verify the voltage at the CO detector itself (which would likely mean temporarily pulling it from the wall). It should be very nearly the same as the battery voltage; there's no reason for there to be much voltage loss on the circuit since it uses only a tiny bit of power.
If they're OK, logically the next step is to replace the alarm unit.
According to the manual linked, the end of life alarm is a beep every thirty seconds, like the "malfunction" alarm (i.e. low voltage). The light flashes are different for the two cases, however: low voltage is red-green-red-green alternating flashing, while end of life is red-red-green-green alternating.
