Confusion About OBD II Monitors

I’ve written more about OBD II readiness monitors than any other subject that I’ve covered at Unfortunately, I don’t think that I’ve done a very good job explaining what they are.

…If the monitor on my car is the problem, how much does it usually cost to get a replacement monitor?

Comment from a reader in response to:
Smog Check OBD II (OBD 2): What are Readiness Monitors?

Is an OBD II monitor bigger than a breadbox?

An OBD II readiness monitor is not something that you can see, touch, smell or taste. You can not repair or replace an OBD II readiness monitor.

If you were told that your vehicle did not pass a smog inspection because your car’s OBD II readiness monitors were not complete, it doesn’t mean that you have a bad monitor.

OBD II readiness monitors are not sensors!

Your car has an engine control module (Computer).  That computer is loaded with software that performs many functions.  Some of those functions, or processes, are referred to as OBD II readiness monitors.

OBD II readiness monitors are software processes that monitor (Test) critical emissions control systems.

These processes (Monitors) are referred to by name of the system(s) that they monitor (Test).

Just Smogs® 7722 Talbert Ave, Huntington Beach, CA

Fuel Management


Comprehensive Component (Shorts, opens, other electrical issues)


Heated Catalyst (Uncommon)

O2 Sensor

O2 Sensor Heater


Secondary Air Injection


Not all of the above monitors will be programmed into every vehicle.  For example, there will be no Secondary Air Injection monitor on a vehicle that is not equipped with a secondary air injection system because there is no secondary air injection system to monitor (test) on that vehicle.

Again, OBD II readiness monitors are tests run by your vehicle’s computer software. OBD II readiness monitors are not physical components or sensors.

…So if you have a ford or chevy with 9 monitors, 8 have to be ready if you have a brand new german car with 100 sensors, 99 have to be ready – how stupid is that….. “1 not ready” is a stupid criteria. My old ’95 only had 2 sensors – so “none” ready would have passed…

Excerpt from a forum post at

The author of that post is a confused about OBD II monitors, but that’s OK, so are many automotive professionals.

Also, there is not a 1:1 ratio between monitors and sensors. Even if a gasoline powered vehicle is equipped with “100 sensors”, It will have no more OBD II readiness monitors than those listed above.

Why are OBD II Readiness Monitors Important?

When your car’s computer detects a problem that could cause an increase in harmful emissions (Smog), it will set a diagnostic trouble code (DTC) and turn on the “Check Engine Light”.

The check engine light is also known as the Malfunction Indicator Light (MIL), Service Engine Soon, and the funny looking thing with the lightning bolt going through it).

The computer identifies problems by running the diagnostic tests that we call monitors.

If no diagnostic trouble codes are stored in memory and all OBD II readiness monitors are complete, it is safe to assume that the vehicle’s emissions control systems are working properly; however, if the monitors are incomplete, there might be a problem with the emissions control system that has not been identified.

Since OBD II testing and system health is a major component of today’s smog inspection, a vehicle can not pass a smog inspection unless all systems are ready.

If they’re not broke, then why aren’t my monitors ready?

By design, OBD II monitors are composed by tests that run in the background and the average motorists never has to give OBD readiness monitors a second thought.

OBD readiness monitors are normally “Ready” (Complete). If OBD II monitors are “Not Ready” (Incomplete), it is usually because of one of the following reasons:

The computer lost power

The results of tests run by OBD II readiness monitors are stored in what is known as volatile memory.

Volatile memory, contrary to non-volatile memory, is computer memory that requires power to maintain the stored information; it retains its contents while powered on but when the power is interrupted, the stored data is lost immediately or very rapidly.

That means that if the computer in your vehicle looses power for any reason, the results of tests run by the computer’s OBD II readiness monitors will be cleared (erased, deleted, etc).

Reasons for a vehicle’s computer to loose power commonly include:

  • A dead or depleted battery.
  • A battery that has been disconnected.
  • Electrical problems that could include blown fuses and damaged wiring.

One of my customers had an ignition interlock device installed on her vehicle after she plead guilty to a DUI. The device installed on her vehicle worked by interrupting the power supply to her car’s computer.

Because of that interlock device, the OBD II readiness monitors on her car were cleared every time she turned switched off the ignition on her car.

When power returns to the computer all monitors will indicate a “Not Ready” or “Incomplete” state (The term used will depend on the diagnostic scan tool used to check monitors).

Cleared with a diagnostic scan tool

When a car comes into a shop with an illuminated check engine light (MIL), the vehicle should be properly diagnosed and repaired.

In some cases the check engine light may be cleared as part of the diagnostic process.  A technician might gather relevant information, and then verify that the problem still exists by clearing the code and confirming that the code returns under similar conditions.

In any case, the check engine light is usually cleared following the completion of repairs.

By design, when codes are cleared using a diagnostic scan tool or code reader, OBD readiness monitors are also cleared.

Most shops will instruct their customer to drive the car normally, and return to the shop only if the original symptoms, including the illuminated check engine light, return. In most cases this isn’t a problem, but what if the customer leaves the repair shop and drives straight to a smog shop? That customer might have a problem.

So, what do I have to do to get my monitors ready?

Drive your car!

I won’t begin to describe some of the silly and outright dangerous schemes that people come up with to avoid driving their own cars.

Each monitor (Test) has a specific set of conditions (Enabling criteria) that must be met before that monitor can be run to completion.

The best way to complete readiness monitors, especially on car that is having trouble completing those monitors, is to obtain a copy of the manufacturer’s suggested drive cycle. In most cases, that drive cycle is only an internet search away.

I can’t do that in Los Angeles/Orange County traffic!

At Just Smogs® in Huntington Beach, we frequently perform OBD II readiness drive cycles for our customers during normal business hours.

Earlier this month, a customer from Temecula  brought us a Jeep with “impossible to run monitors”.  After allowing the Jeep to fully cool down, one of my team members warmed up the Jeep and took it out for a spin. He was back in less than an hour. All monitors were complete.

Here are some tips for running monitors and difficult vehicles:

  1. Follow all manufacturer instructions, not just the ones you like.  Deviating from the drive cycle instructions briefly can take you back to square one.
  2. I often recommend running monitors early on a Saturday or Sunday morning when traffic is usually light. Also, some monitors will not complete during extremely hot or cold weather (Usually the EVAP monitor).
  3. In most cases, allow the vehicle to cool down completely overnight. It’s usually important to perform a complete warm up cycle before beginning the drive cycle.  Allow the vehicle to warm up naturally while idling. Unless the drive cycle instructions indicate otherwise, avoid revving the engine in an effort to warm it up quickly.
  4. Once again, follow the manufacturer’s instructions completely!

I’ve done all that, and they still wont run!

There are times when the drive cycle will not work!

Normally a problem with a monitored system, sensor,  or other component, will trigger a diagnostic trouble code and the computer will turn on the check engine light.

However, sometimes an under-performing system that is on the verge of failure will prevent the completion of a test, but not trigger a diagnostic trouble code.

For example an under-performing oxygen sensor might prevent the completion of a catalyst monitor, or an exhaust leak might affect an oxygen sensor’s performance just enough that it prevents the monitor from running to completion. Even though the check engine light might be off, a monitor might not complete until necessary repairs are performed.

Some vehicles have known software or hardware issues that prevent monitors from being run to completion. In those cases the vehicles computer may require reprogramming or  replacement.  The vehicle may even be subject to a manufacturer recall.

More information about OBD II monitors and problem vehicles can be found in the OBD II Smog Check OBD Reference maintained by the California Bureau of Automotive Repair.

Just Smogs® in Huntington Beach specializes in the diagnosis and repair of emissions related issues including OBD II monitor issues.  For information about diagnostics, repair, or the Just Smogs® monitor drive cycle service, call Just Smogs® at (714) 596-1019.


BAR OIS Testing And Invalid Monitor Information

On August 1, 20016, the Bureau of Automotive Repair (BAR) implemented a change in the BAR-OIS testing procedure that may result in a new type of smog inspection failure.

What is BAR-OIS testing?

just-smog-check-huntington-beach-3-bays-800x480The BAR-OIS is the Smog Check equipment required when inspecting most model-year 2000 and newer gasoline and hybrid vehicles and most 1998 and newer diesel vehicles. The system consists of a certified Data Acquisition Device (DAD) and off the shelf equipment, including a computer, bar code scanner, and printer.

During a BAR-OIS smog inspection, the DAD collects data from your vehicle’s Power train Control Module (PCM), or as most people call it, “The Computer”.

The data collected by the DAD includes diagnostic trouble code information (Mode $03), Vehicle Information (Mode $09), and the current monitor information from Mode $01. Other data is also collected, but for the purpose of this article, we’ll focus on Mode $01 monitor information.

OBD II Monitor Readiness

Image of instrument cluster including "Check Engine" lightOBD II Monitor Readiness is a key element of the BAR-OIS smog inspection process.

Most model year 2000 and newer gasoline powered and hybrid vehicles can pass the BAR-OIS inspection if EVAP is the only incomplete monitor; otherwise, all other OBD II monitors must be complete.

While most vehicles behave as expected, some vehicles do not respond with proper OBD monitor information from the PCM, or the transmission computer responds instead of the engine computer.

In most cases cycling the vehicle’s ignition (off/on) will solve the problem, but occasionally the vehicle will continue to respond with invalid or no data.

Prior to August 1, 2016, a vehicle could pass the BAR-OIS inspection if this condition continued; however, that is no longer the case.

What has changed?

toyota-drive-cycle-obd-readiness-monitor-smog-checkEffective August 1, 2016, if a vehicle continues to respond with improper OBD II monitor information, the vehicle will fail the inspection.

If a vehicle in the following list fails for invalid OBD II monitor information, it should be referred to the BAR Referee. Vehicles not included in the list will most likely require repairs.

  • 2004 Volvo C70 HPT
  • 2004 Volvo C70 LPT
  • 2004 Porsche Boxster S
  • 2004 Porsche Boxster
  • 2003 Porsche Boxster