Readiness monitors are self-tests performed by the vehicle's OBD system to verify emission control functionality.Checked 2026-08-12
Readiness monitors
Self-tests the OBD-II system must complete before an emissions test can be read.
Also called: OBD readiness monitors · monitor status · not ready monitors
What it means
Definition
California's Bureau of Automotive Repair describes readiness monitors as self-tests performed by the vehicle's OBD system to verify emission control functionality. Each one needs particular driving conditions before it can finish, so until it does its status reads not ready rather than pass or fail. CARB notes that after an OBD system has been reset or cleared it takes time and operation to bring the system back into a fully ready state.
Why a buyer cares
What it changes
Monitors reset when codes are cleared or the battery is disconnected. A car presented for sale with monitors not ready is a car whose recent code history cannot be read yet.
In practice
Where you meet it
On a scan-tool readout and on an emissions inspection report, listed monitor by monitor as ready, not ready or not applicable.
Two things that look alike
Not the same as
A not-ready monitor is an unfinished test, not a fault. It says nothing about whether a problem exists.
Read next
Related terms
OBD-II (On-Board Diagnostics II)
The standardised self-diagnostic system in modern cars, read through a connector under the dash.
Diagnostic Trouble Code (DTC)
The code an OBD-II system stores to identify which monitored system it found a fault in.
Emissions inspection
A state or local test of a vehicle's emissions performance, often required to register it.
Where this comes from
Sources
If a vehicle's OBD system has been reset or recently cleared, also referred to as "code clearing," it will take time and operation to bring the vehicle's OBD system back into a fully ready state; i.e., all vehicle OBD monitors in the ready state meaning the OBD system has operated sufficiently to determine the presence or absence of a DTC.Checked 2026-08-12
The second quote comes from CARB's heavy-duty Clean Truck Check page. The mechanism matches light-duty OBD-II per the BAR source, but a light-duty CARB or EPA sentence on the reset-to-ready sequence was not found.