OBD Readiness Monitors Explained

Once in a while we get questions about OBD Readiness Monitors. This post will explain what the readiness monitors are.

The purpose of readiness monitors in a car is to self-test the car’s emission systems. Readiness monitors are self check routines that observe the performance of specific vehicle emissions control systems. Cars may perform up to 11 system tests; these are so called readiness monitors. The output of readiness monitors identify whether the car’s computer has completed the required tests while the car is being driven.

Readiness Monitor types

There are two different types of monitors: continuous and non-continuous. Continuous monitors are different in design from the non-continuous monitors. Continuous monitors are being constantly tested and evaluated by the car’s computer while the car is running. Conversely, the non-continuous monitors require certain conditions to be met before a test or series of tests can be completed. The conditions necessary for the car to run these self-diagnostic tests vary. Some monitors require that the car follows a predefined “drive cycle” routine. Some require two drive cycles due to the need for a cool down and warm up periods in between.

Continuous Monitors

  • Misfire
  • Fuel System
  • Comprehensive Component


Continuous readiness monitors read with the Windows Phone app
Continuous readiness monitors read with the Windows Phone app


Non-Continuous Monitors

Non-continuous monitors are different for spark ignition cars (gasoline engines) and compression ignition cars (diesel engines).


Spark ignition vehicles (Gas)
  • Catalyst (CAT)
  • Heated Catalyst
  • Evaporative (EVAP) System
  • Secondary Air System
  • Oxygen (O2) Sensor
  • Oxygen Sensor Heater
  • EGR (Exhaust Gas Recirculation) and/or VVT System
Compression ignition vehicles (Diesel)
  • NMHC Catalyst
  • NOx/SCR Aftertreatment
  • Boost Pressure
  • Exhaust Gas Sensor
  • PM Filter
  • EGR and/or VVT System


Non-continuous readiness monitors read with the Android app
Non-continuous readiness monitors read with the Android app


Monitor status

Each readiness monitor will have it’s own output status. The completion status can be:

  • Ready or complete (green mark) meaning that the test has been completed e.g. the OBD-II system has checked this emissions control system.
  • Not ready (red mark) meaning the test is uncompleted e.g. the OBD-II system has not checked this emissions control system.

OBD Auto Doctor reports the status only for supported monitors in the mobile apps. The desktop version (Windows, Mac, Linux) lists the unsupported monitors too. They are marked as unsupported. It simply means that the car doesn’t have that monitor and therefore it can’t be tested.


OBD readiness monitors read with the Mac version
OBD readiness monitors read with the Mac version


Monitor “not ready”

Clearing the diagnostic trouble codes (DTCs) will reset the monitor statuses. This typically occurs during vehicle repair. Statuses are reset in case of power failure too. This usually happens when the battery has been disconnected. Therefore it is not advisable to disconnect the battery. If you need to disconnect the battery for example to replace it, read further to learn how to get the monitors back to complete.

Note! Depending on your country and state, OBDII vehicle may not pass the annual inspection unless the required monitors are “ready”. For example, the US Environmental Protection Agency guidelines allow up to two monitors to be in a “not ready” state for model year 1996 through 2000 vehicles and one monitor “not read” for 2001 and newer model year vehicles.

How to get the monitors “ready”?

  1. First, make sure that the MIL (Malfunction Indicator Light) is not commanded on. Having stored or even pending diagnostic trouble codes active may prevent a monitor from running to completion.

  2. Second, make sure that you have enough fuel in the car. Some monitors, for instance the EVAP monitor, may require the fuel level to be between 35% and 85% to initiate the diagnostic testing.

  3. Third, complete the so called “drive cycle”. About one week of combined city and highway driving is usually enough to allow the monitors to reach complete status. The drive cycle is explained in more details in the next paragraph.

OBD drive cycle

The purpose of the OBD2 drive cycle is to let your car run on-board diagnostics. This, in turn, allows monitors to operate and detect potential malfunctions of your cars’s emission system. The correct drive cycle for your car can vary greatly depending on the car model and manufacturer. Also, the monitor in question affects the required drive cycle.

Today, many vehicle manufacturers include these drive cycles in the vehicle owner’s manual. Typically, a few days of normal driving, both city and highway, will make the monitors ready. The following generic drive cycle can be used as a guideline if a specific drive cycle is not known. It will assist with resetting monitors when a car specific drive cycle is not available. However, it may not work for all cars and monitors.

The drive cycle can be difficult to follow exactly under normal driving conditions. Therefore, it is better to drive it in restricted area!

  1. The universal OBD-II drive cycle begins with a cold start (coolant temperature below 50 C /122 F, and the coolant and air temperature sensors within 11 degrees of one another). This condition is easily achieved by letting the car to sit overnight.

  2. The ignition key must not be left on prior to the cold start. Otherwise the heated oxygen sensor diagnostic may not run.

  3. Start the engine and idle the engine in drive for two and half minutes, with the A/C and rear defroster on if equipped.

  4. Turn the A/C and rear defroster off, and accelerate to 90 km/h (55 mph) under moderate, constant acceleration. Hold at a steady speed for three minutes.

  5. Decelerate (const down) to 30 km/h (20 mph) without braking or depressing the clutch for manual transmissions.

  6. Accelerate back to 90-100 km/h (55-60 mph) at 3/4 throttle. Hold at a steady speed for five minutes.

  7. Decelerate (const down) to a stop without braking.

Get ready for inspection

To avoid being rejected in the annual inspection, you can prepare your car for it yourself. Do not wait until the annual inspection with your issues. If the check engine light comes on, read the diagnostic trouble codes and engine status immediately. It could save you a lot of time as well as future repair and fuel costs. You can do all this with OBD Auto Doctor diagnostic software. You can read all the monitors statuses even with the free version. Try the software now!




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