After clearing fault codes, or after a battery disconnection, the warning light may go off but the vehicle will fail an emissions inspection until its readiness monitors have run to completion. “Not ready” or “incomplete” readiness monitors are one of the most common reasons a vehicle is turned away at an emissions test station. Understanding what the monitors check, what it takes to complete them, and how to handle monitors that will not set is a practical skill for any diagnostic or service workflow.
What readiness monitors are
OBD-II readiness monitors (also called I/M readiness monitors — from Inspection and Maintenance) are self-tests that the PCM runs during normal driving to confirm that emissions-related systems are functioning correctly. Each monitor has specific enable criteria — temperature ranges, load conditions, speed thresholds, and timing requirements that must be met before the PCM considers the test valid. When the test completes successfully, the monitor is set to “ready” or “complete.” When the monitor has not yet run since the last code clear, it reports “not ready” or “incomplete.”
The OBD-II standard defines a set of continuous monitors that run whenever the engine is on, and a larger set of non-continuous monitors that run only under specific operating conditions.
Continuous monitors
These run constantly while the engine is operating and do not need specific driving conditions to complete. They reset immediately after a code clear and resume monitoring without a drive cycle requirement.
- Misfire monitor: Counts crankshaft deceleration events caused by incomplete combustion. Fires misfire codes (P0300–P0312) when the rate exceeds threshold. Runs continuously at all engine speeds and loads.
- Fuel system monitor: Watches short-term and long-term fuel trims. Detects lean or rich conditions that exceed the acceptable correction range. Fires P0171/P0172 and similar codes. Runs whenever the engine is in closed-loop fuel control.
- Comprehensive component monitor (CCM): Checks sensors and actuators for electrical circuit faults — out-of-range voltages, open circuits, shorts. Covers most sensors that report to the PCM. Runs continuously.
Non-continuous monitors
These run only once per drive cycle when their specific enable conditions are met. They are the monitors that go “not ready” after a code clear and require a proper drive cycle to reset.
| Monitor | What it checks | Typical enable conditions |
|---|---|---|
| Catalyst (CAT) | Catalyst efficiency by comparing upstream and downstream O2 sensor activity | Warm engine, extended steady cruise at moderate speed and load |
| Evaporative system (EVAP) | Fuel system for vapor leaks (large and small) | Cold start from below ~86°F (30°C) soak temp; specific warm-up and idle sequence |
| Oxygen sensor (O2S) | O2 sensor response time, switch frequency, and voltage range | Warm engine, closed-loop fuel control, transitions between load conditions |
| Oxygen sensor heater (HTR) | Heater circuit resistance and current draw | Cold start; heater must cycle on and warm sensor to operating temperature |
| EGR system | EGR flow rate and valve response | Engine warm, light to moderate load, specific throttle actuation sequence |
| Secondary air injection (AIR) | Air pump output and check valve function | Cold start; pump commanded on during cold fast idle |
| Positive crankcase ventilation (PCV) | Not a separate monitor on most platforms — tested via CCM for circuit faults | Varies; some platforms test PCV hose integrity via EVAP monitor |
| Air conditioning refrigerant monitor | A/C refrigerant charge level via high-side pressure vs ambient temperature | A/C on, specific temperature and pressure conditions |
| Thermostat monitor | Coolant warm-up rate — confirms thermostat is not stuck open | Cold start below ~86°F (30°C); ECT must reach operating temp within specified time |
| Boost pressure monitor (turbocharged) | Boost control solenoid response and maximum boost achieved | Warm engine, full-throttle acceleration |
Not every vehicle uses every monitor. A non-turbocharged vehicle will not run the boost pressure monitor. A vehicle without secondary air injection skips that monitor. The monitors that are “not applicable” are reported as complete automatically — they do not need to run.
How many monitors can be incomplete at an emissions test?
OBD-II inspection regulations specify allowable monitor incompleteness based on model year:
- 1996–2000 vehicles: Typically two non-continuous monitors may be incomplete and the vehicle will still pass.
- 2001 and newer vehicles: Typically only one non-continuous monitor may be incomplete. The EVAP monitor is most commonly the one that does not reset cleanly — some states allow EVAP to be incomplete on 2001+ vehicles as a specific exception, but many do not. Check your local regulations.
If any monitor reports “not ready” that is not within the allowable incomplete count, the vehicle fails regardless of whether the MIL is off and no codes are stored.
How to complete the monitors: the general OBD-II drive cycle
EPA specifies a general OBD-II drive cycle that is intended to complete most non-continuous monitors. It is not guaranteed to complete all monitors on all vehicles — each manufacturer implements their own enable criteria — but it addresses the most common monitors on most platforms.
- Cold soak requirement. For EVAP and thermostat monitors, the vehicle must have been off and at ambient temperature for at least 8 hours — ideally overnight. Ambient temperature should be above 40°F (4°C). This is non-negotiable for EVAP on most platforms.
- Cold start. Start the engine from cold without pressing the accelerator. Idle for 2–3 minutes. This runs the oxygen sensor heater monitor and starts the cold-start enable window for EVAP and thermostat.
- Moderate acceleration. Accelerate at moderate throttle to 55 mph (90 km/h). Avoid wide-open throttle. This allows fuel system and O2 sensor monitors to enter their operating windows.
- Steady highway cruise. Hold 55–60 mph (90–100 km/h) for 3–5 minutes without significant throttle variation. The catalyst monitor requires a sustained steady-state cruise with the engine in closed-loop control — this is typically the longest requirement.
- Deceleration without braking. Release the throttle and let the vehicle decelerate from 55 to 20 mph (90 to 30 km/h) without using the brakes. This triggers the fuel cut that helps complete the catalyst and O2 sensor monitors.
- City driving segment. Drive at 15–25 mph (25–40 km/h) with moderate acceleration and deceleration, several stops and starts. 5–10 minutes is typically sufficient. This helps complete the EGR monitor on applicable vehicles.
- Rescan for readiness. Return to the service area and check readiness monitor status. Repeat the highway and city segments if monitors remain incomplete.
Platform-specific drive cycles
The general drive cycle is a starting point. Manufacturers publish their own drive cycle procedures for vehicles where the general cycle is insufficient. If monitors are still incomplete after a general cycle:
- Look up the OEM drive cycle procedure for the specific platform in service data — Toyota, Honda, Ford, GM, and VAG all publish detailed sequences with speed, load, and temperature requirements.
- The EVAP monitor is the most common holdout. It typically requires a very specific cold-soak temperature and an exact warm-up sequence. Some Ford and GM platforms have documented EVAP “trips” that differ significantly from the general EPA cycle.
- The catalyst monitor on some Honda platforms requires a specific engine speed and load combination sustained for longer than the EPA cycle specifies.
- Some scan tools include a “drive cycle assistant” that monitors PIDs in real time and tells you when each enable condition is satisfied — this removes the guesswork from the process significantly.
Monitors that will not set: common causes
If a monitor remains incomplete after multiple proper drive cycles, there is usually a reason:
- Active fault that prevents the monitor from running: Some monitors will not run if a related fault code is stored or pending. A pending O2 sensor code blocks the catalyst monitor. An active EVAP circuit code blocks the EVAP leak test. Resolve the fault first.
- Enable conditions not being met: Temperature too warm for EVAP cold-start requirement, elevation affecting boost monitor results, ambient conditions outside the monitor’s operating range. Check the specific enable criteria in service data.
- Marginal system that passes the monitor then immediately trips it again: The monitor completes and sets “ready,” but the system is close enough to the limit that it trips a pending or stored code and resets the monitor on the same drive. This looks like the monitor will not hold — it is actually running and setting a code you may not have noticed.
- Monitor disable from a non-emissions fault: Some platforms disable specific monitors when a non-related fault is active. A throttle body fault can disable the EGR monitor on some platforms. Check all stored and pending codes before assuming a monitor drive cycle failure.
Frequently asked
How long does it take to complete all monitors?
On most vehicles, a properly executed drive cycle takes 20–40 minutes of actual driving following the overnight cold soak. The EVAP monitor is typically the last to complete and has the most demanding cold-start requirements. Plan for at least two complete drive cycles on vehicles where EVAP is the only incomplete monitor.
Do readiness monitors reset when the battery is disconnected?
Yes. Battery disconnection clears the keep-alive memory where monitor completion status is stored. The PCM starts fresh with all non-continuous monitors showing “not ready.” This is one reason why disconnecting the battery before an emissions test to turn off the MIL will not work — the monitors are also cleared, and the vehicle will fail for incomplete monitors even if no codes are stored.
The MIL is off and no codes are stored. Why did I fail the emissions test?
The most common reason is that the codes were recently cleared — either by a technician or by the battery being disconnected — and the monitors have not had time to complete. Some shops clear codes immediately before sending a customer to an emissions test in hopes the light does not come back on. The monitor completion status exposes this — the inspection station can see that the monitors are all “not ready,” which is consistent with a recent code clear. A full OBD-II drive cycle is required to restore monitor completion.
Can I check readiness monitors myself?
Yes. Any OBD-II compatible scan tool — including basic code readers — can display readiness monitor status. This is standard OBD-II Mode $01 and Mode $06 data that all compliant tools can access. Check monitor status before taking the vehicle for an emissions test to avoid a failed inspection visit.