OBDII Monitor Readiness Concerns

Modern vehicles perform various self-tests (Monitors) in order to ensure the OBDII and emission control systems are functioning properly. In many cases, Monitor Readiness problems can cause the vehicle to fail a smog check. Note that it is possible for a vehicle to fail the smog check for Monitor Readiness even though the MIL or Check Engine light is not on and there are no obvious indicators of a problem. Despite being driven hundreds of miles after failing the test, many vehicles will not run all the Monitors until a thorough diagnosis and repair is performed. Most vehicles need to complete a specific set of enabling criteria (Drive Cycle) in order to cause a Monitor to run. Note that Readiness Monitors must be rerun after a repair like disconnecting a vehicle’s battery or replacing a defective emissions component. 

In some instances after following instructions to “drive the car” to correct the Monitor problem, consumers are again instructed to simply “drive the car more,” or are incorrectly referred to the Referee to “override the problem.” If you are given these vague or inaccurate instructions, you may wish to have your vehicle diagnosed at another qualified repair facility that is more familiar with OBDII Readiness Monitor and Drive Cycle issues. While there are some exceptions, the overwhelming majority of vehicles will run all the Monitors provided it has been properly diagnosed and repaired.

The number of incomplete OBD readiness monitors that are allowed to pass the OBD test portion of a Smog Check inspection are provided below.

Model YearFuel TypeNumber of Incomplete Monitors Allowed to Pass OBD Test1
1996-1999Gas2Any one
2000 and newerGas2Evaporative system
2007 and newerDieselParticulate Filter and Non-Methane HydroCarbon


  • For more information on OBDII Monitor Readiness Standards, visit OBD Reference.  
  • For more general information on the Referee program, visit Referee Program.