Diagnostic readiness monitors are the self-tests your car’s computer runs to confirm that its emissions control systems are working as designed. Every OBD-II vehicle sold in the United States since model year 1996 carries them, and their results decide much of what a scan tool can tell you.
This guide explains what each monitor checks, why some run constantly while others wait for specific conditions, and how to read their status on an ordinary code reader. You will also learn how monitor results connect to trouble codes and how to use them before deciding on any repair.
Your Car Is Always Grading Its Own Emissions Systems
Modern engines are managed by a powertrain control module, often called the PCM or ECM. Besides running fuel and spark, this computer watches the parts that keep exhaust emissions low, including the catalytic converter, oxygen sensors, and the evaporative system that traps fuel vapor.
OBD-II, short for On-Board Diagnostics, second generation, standardized how that watching is reported. Instead of each manufacturer hiding results in proprietary formats, every compliant vehicle shares a common set of monitor results through the 16-pin diagnostic connector, usually found under the dashboard on the driver’s side.
For an owner, this means a basic scan tool can show whether the car has finished testing itself. That single piece of information explains why a car can fail an emissions inspection with no warning light on, and why a fresh repair sometimes cannot be confirmed right away.
What a Readiness Monitor Actually Is
A readiness monitor is a diagnostic routine stored in the engine computer. It tests one system or component against limits set by the manufacturer, then records whether the test has run to completion since the memory was last cleared. Completion and pass are related, but they are not the same thing.
When a monitor finishes and finds no fault, it reports complete. If it detects a problem, it can set a pending trouble code and, after repeated failures, a confirmed code that turns on the malfunction indicator lamp, the amber engine symbol on the dash.
The word readiness comes from emissions testing. Inspectors want to know whether the car is ready to be judged, meaning its self-tests have actually run. A vehicle whose monitors have not finished cannot prove it is clean, even if nothing is wrong with it.
Continuous Versus Non-Continuous Monitors
OBD-II divides monitors into two families based on when they run. Knowing the difference explains why some results appear within minutes of starting the engine while others can take days of normal driving to report.
Continuous Monitors That Run All the Time
Three monitors run continuously whenever the engine operates: misfire, fuel system, and comprehensive components. The misfire monitor watches crankshaft speed for the stumbles that misfires cause. The fuel system monitor tracks how much the computer must correct the air-fuel mixture. The comprehensive component monitor checks sensors and actuators for circuit faults and implausible readings.
Because they never stop working, these monitors usually show complete on a scan tool almost immediately after codes are cleared.
Non-Continuous Monitors That Need Specific Conditions
Other monitors run only once per trip, and only when conditions are right. The catalyst, oxygen sensor, oxygen sensor heater, evaporative system, and EGR monitors may need a fully warm engine, a certain fuel level, steady cruising, or a long idle. Until those conditions occur, the monitor stays incomplete.
This waiting is deliberate. Running a catalyst or evaporative test under the wrong conditions could produce false failures, so the computer holds the test until it can trust the result.
Diesel Monitors Are a Different Set
Diesel vehicles report their own non-continuous monitors, such as those for the NOx aftertreatment, particulate filter, boost pressure, and exhaust gas sensors. The reporting method is the same, but the list differs from a gasoline engine. A scan tool displays whichever set the vehicle supports, so owners should not expect identical lists across fuel types.
The Monitors You Will See on Most Gasoline Cars
The exact list depends on what equipment a vehicle carries, but most gasoline cars and light trucks report a familiar group of non-continuous monitors. Each one targets a system that can raise emissions when it fails.
- Catalyst: compares upstream and downstream oxygen sensor signals to judge how well the converter stores oxygen.
- Oxygen sensor: checks that sensors switch and respond quickly enough to mixture changes.
- Oxygen sensor heater: confirms the heating elements bring sensors to working temperature.
- Evaporative system: tests the fuel vapor system for leaks and proper purge flow.
- EGR or VVT: verifies exhaust gas recirculation or variable valve timing operates as commanded.
- Secondary air: checks the air injection system on engines that use one.
Some vehicles also list a heated catalyst or air conditioning refrigerant monitor, although few cars use them. A monitor marked not supported simply means the vehicle lacks that hardware, which is normal and is generally not counted against the car during an inspection.
Reading Monitor Status With a Scan Tool
Almost every OBD-II code reader can show readiness status, sometimes labeled I/M readiness, where I/M stands for inspection and maintenance. The process takes only a few minutes and does not change anything stored in the computer.
- Park on level ground outdoors or in a ventilated space and set the parking brake.
- Locate the 16-pin connector under the driver’s side of the dashboard and plug in the tool.
- Turn the ignition on, or start the engine if the tool’s instructions call for it.
- Open the readiness or I/M menu and wait for the tool to finish communicating.
- Record each monitor’s status and the reported MIL condition before unplugging.
Write the results down or photograph the screen. Comparing readings taken a few days apart is the simplest way to see whether incomplete monitors are progressing, and that record is helpful if you later need to explain the vehicle’s history to a technician.
Complete, Incomplete, and Not Supported Explained
Scan tools use slightly different wording, but every readiness screen reports three possible states. Complete, sometimes shown as ready or OK, means the monitor has run its test since memory was last cleared. Incomplete, shown as not ready or INC, means the test has not yet finished.
Not supported, often shown as N/A, means the vehicle has no such system to monitor. It is not a warning. A car without secondary air injection will always display that monitor as not supported, and that is exactly what the computer should report.
Be careful with the word complete. On many vehicles, a monitor that ran and failed can still show complete, because the test did finish. The failure is recorded as a pending or confirmed trouble code instead, so you must always check for codes alongside readiness status.
How Monitors Relate to Trouble Codes
Monitors and trouble codes are two halves of one system. The monitor is the test, and the diagnostic trouble code, or DTC, is the record left behind when the test fails. A code such as P0420, catalyst efficiency below threshold bank 1, comes from the catalyst monitor.
Most non-continuous monitors use two-trip logic. A first failure stores a pending code without lighting the lamp. If the same fault appears on a later qualifying trip, the code becomes confirmed and the malfunction indicator lamp turns on.
This is why a code points to a fault the monitor detected, not to a specific part. A P0420 might trace to the converter, but it can also follow an exhaust leak or a faulty oxygen sensor reading. Test before replacing parts, and let the monitor confirm the fix afterward.
Why a Monitor Refuses to Run
When one monitor stays incomplete for weeks, there is usually a reason. Many monitors refuse to run while another fault is present, because a bad input could make their results unreliable. A misfire or oxygen sensor code, for example, can block the catalyst monitor from testing.
Operating conditions also matter. Evaporative monitors on many vehicles need a fuel level within a certain range and a cold soak, meaning the car must sit for hours before starting. Short trips that never fully warm the engine can keep several monitors waiting indefinitely.
The enabling conditions are set by each manufacturer and are not universal. Your owner’s manual, the factory service information, or a qualified technician can tell you what a specific monitor on your vehicle needs. Avoid relying on generic advice that promises one routine fits every car.
Common Mistakes When Interpreting Monitor Status
The most frequent mistake is treating an incomplete monitor as a failure. Incomplete only means the test has not finished. Replacing parts because a monitor shows not ready usually wastes money and leaves the real question, whether the system works, still unanswered.
A second mistake is clearing codes right before checking readiness. Clearing resets most monitors to incomplete, so the screen will show a car that looks unprepared even if every system was healthy. Read status first, and clear codes only after a confirmed repair.
A third mistake is ignoring the MIL status line on the readiness screen. That line reports whether the computer is commanding the warning lamp on. If it reads on while the dashboard bulb is dark, the instrument cluster or bulb circuit needs attention as well.
Using Monitor Status to Plan Your Next Step
Readiness status is most useful as a planning tool. Before an emissions inspection, it tells you whether the car can be judged. After a repair, it tells you whether the system has been retested. Before buying a used car, it can reveal that codes were recently cleared.
If several monitors are incomplete and no codes are stored, normal mixed driving over the following days is usually the next step. Check status again afterward. If one monitor stays incomplete while the others finish, research its enabling conditions or ask a technician to look for a blocking fault.
When a monitor completes and codes return, the computer has done its job and confirmed a problem. At that point, use the stored code, freeze frame data, and proper testing to locate the fault. Monitors are the starting point for diagnosis, not a substitute for it.