Emissions Readiness Monitors

Emissions monitors have been a source of confusion and headache for many drivers trying to get their vehicle to pass the Smog Check. Many people ask what they are, the Bureau of Automotive repair has this to say about them:

“Readiness monitors are self-tests performed by the vehicle’s OBD system to verify emission control functionality. While most vehicles complete the various readiness monitors during normal driving, other vehicles have more trouble either because the vehicle design requires unusual operating conditions or because the owner rarely drives the car in the necessary operating mode. On occasion, the vehicle owners-manual contains driving procedures to complete the monitors, but usually this drive cycle information is only available to the OEM or found in repair industry service literature. Since drive cycles require the vehicle to experience specific operating conditions: speed, temperature, pressure, engine load, etc., performing certain drive cycles may not be possible in traffic, yet could be performed using dynamometer equipment. Monitor completion is also dependent on properly functioning components, so a vehicle may never complete a monitor until a thorough diagnosis and repair of components enabling the monitor to run is performed. Readiness monitors must be rerun after a repair activity like disconnecting a vehicle’s battery or replacing a defective emissions component.”

The typical gas engine monitors are:

  • Misfire
  • Fuel system
  • Comprehensive Component
  • Catalyst (CAT)
  • Heated Catalyst
  • Evaporative (EVAP) System
  • Secondary Air System
  • Oxygen (O2) Sensor
  • Oxygen Sensor Heater
  • EGR (Exhaust Gas Recirculation) and/or VVT System


Diesel engine monitors can include:

  • NMHC Catalyst
  • NOx/SCR Aftertreatment
  • Boost Pressure
  • Exhaust Gas Sensor
  • PM Filter
  • EGR and/or VVT System


Most vehicles complete the monitors to be ready for a Smog check in as few as 10-20 miles, others take much longer. We have seen some diesel pickups and vans take nearly 2000 miles to complete.

When a vehicle has incomplete monitors preventing it from passing the Smog Check the OBD Inspection System will issue a “Not Ready” result. What to do next often varies based on the situation. If the vehicle very recently had repairs or a dead battery it typically just needs to be driven through various conditions. Depending on the drivers’ habits this usually only takes about a week.

Below is a very generic OBD drive cycle that will help complete the monitors on most vehicles.  This is not exact for all vehicles, but it does need to be done in this order and in a single trip. Some vehicles will require this cycle to be completed 2 or more times.

  1. Let the vehicle sit for 8 hours
  2. Start the vehicle and let it warm up (approx. 5 minutes)
  3. Drive for about 10 minutes at highway speeds (50-65 mph)
  4. Drive for about 20 minutes in urban traffic


Some vehicles are much more difficult to complete the emissions monitors on and can take weeks if driven normally. Many of these vehicles have very specific conditions that must be met to allow the tests to run and complete. In some cases, our technicians can drive the vehicle with a scan tool while monitoring the vehicle’s data and keep it within those parameters to help the monitor complete.

If your gas engine vehicle has not completed all of its monitors in 200-300 miles then it is likely there is a very specific criteria that is not being met or something is preventing the monitor(s) from completing. Sometimes vehicles have software problems preventing monitors from completing to ready and the manufacturers often put out updates to correct these issues. It Is also possible that a component or sensor on the vehicle is failing but not yet to the point of setting a trouble code that will make the malfunction indicator lamp to turn on.

One thing that makes completing emissions monitors difficult is that so many vehicles have different criteria. For example, we have found it easiest to run Ford catalyst monitors in stop and go traffic, but GMC/Chevrolet catalyst monitors typically run at idle after driving on the highway. Some GM air injection monitors run at start up, but others must be driven between 15-20 mph. Many Dodge/Jeep/Ram/Chrysler vehicles have updates available to help monitors complete properly. We have seen quite a few late model Honda/Acura and Audi/VW vehicles require hard acceleration to complete the EGR/VVT monitor, so if you are a driver who takes it easy around town it might be a good idea to take the car out on the freeway and make a couple of hard accelerations. Toyota/Lexus and Nissan/Infiniti vehicles typically don’t have issues completing any of their readiness monitors.

If you have a vehicle that is not passing its California Smog Check due to incomplete emissions monitors in normal driving then it may be time to have one of our skilled technicians look into what the issue may be. Feel free to schedule an appointment by phone or online here: https://www.alltechauto.com/appointments

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