Car Diagnostics

Readiness Monitors

Readiness monitors are self-tests that the ECU runs during normal driving to verify that key systems are working correctly: catalyst, oxygen sensors, EVAP system, EGR, and others. Each test result is either Ready (test passed) or Not Ready / Incomplete (test not yet run).

Why monitors matter

Emissions testing and inspection. In many jurisdictions, vehicle inspections include an OBD-II connection to check monitor status. If more than one monitor (two for pre-2000 vehicles) is “Not Ready”, the car fails inspection — even with no stored fault codes.

After clearing codes. When you reset DTCs (Mode 04) or disconnect the battery, all monitors reset to “Not Ready”. They return to “Ready” only after you complete a specific drive cycle.

Types of monitors

Continuous monitors

These run constantly while the engine is running:

  • Misfire Monitor — detects combustion misfires
  • Fuel System Monitor — checks fuel trim corrections (STFT/LTFT)
  • Comprehensive Component Monitor (CCM) — checks sensors and actuators

Non-continuous monitors (Once-Trip / Two-Trip)

Run only under specific driving conditions:

  • Catalyst Monitor — catalytic converter efficiency (compares upstream/downstream O2)
  • Oxygen Sensor Monitor — O2 sensor response time and accuracy
  • Oxygen Sensor Heater Monitor — checks the lambda sensor heater element
  • EVAP Monitor — fuel vapor recovery system leak test
  • EGR / VVT Monitor — exhaust gas recirculation / variable valve timing
  • Secondary Air Monitor — on vehicles with an air injection pump
  • AC Refrigerant Monitor — on some vehicles

How to check monitor status

  1. Connect your adapter and open the Monitors or Readiness section
  2. You’ll see a list of tests with icons: ✅ Ready / ⚠️ Not Ready / — Not Applicable
  3. “Not Applicable” means the system doesn’t exist on your vehicle (e.g., no EGR or secondary air)

How to complete a Drive Cycle

There’s no universal Drive Cycle for all vehicles, but this sequence satisfies most ECUs:

  1. Cold start (ECT below 120°F / 50°C) → warm up to operating temperature (at least 10 min)
  2. City driving 5-10 min: starts, stops, accelerating to 40-50 mph
  3. Steady highway at 50-75 mph for 5-10 min (Catalyst Monitor and EVAP need steady speed)
  4. Several decelerations with foot off the gas (fuel-cut deceleration)
  5. Idle again for 2-3 min after returning

Some monitors (especially EVAP) need specific conditions (fuel level 35-85%, ambient temp under 95°F at startup). Check your vehicle’s service manual for the exact drive cycle.

What to do if a monitor won’t go Ready

Catalyst Monitor: the catalyst may be worn, or there’s an active P0420/P0430. Monitors won’t complete when there are active codes.

EVAP Monitor: most often fails to run with the fuel cap not fully closed, or with fuel level below 20% or above 90%. Fill to 35-80%, check the cap is tight.

O2 Monitor: check O2 sensor response in Live Data. If the O2 sensor voltage is stuck on a fixed value — the sensor is bad.

Misfire Monitor is always continuous: it runs constantly and doesn’t need a drive cycle. If marked Not Ready — there’s an active misfire happening.