Tips From A Tech: Getting Ready with Readiness Monitors

ASE-certified technician uses a scan tool.

Readiness monitors refer to specific programing within an OBDII system that is designed to run various self-checks for emissions compliance.

If these monitors successfully run and pass, the vehicle is said to be in compliance with federal and state emissions laws. For these monitors to begin their self-check of the vehicle, certain enabling criteria must be met. Once satisfied, a drive cycle must be completed for the monitor to report its results to the PCM.  

Setting readiness monitors can be a frustrating task, to say the least. For your best chance of success, understanding the importance (and differences) of enabling criteria and drive cycles will better ensure an accurate repair, as well as a happy customer.   

Enabling criteria are the conditions that need to be met before a monitor will run. Fuel level, temperature, closed loop status are just a few examples of these conditions. Think of enabling criteria as the “prerequisites” before the PCM allows the monitor to run. What’s tricky is that there are no set standards for enabling criteria. Not only do they vary from manufacturer to manufacturer, but can be specific right down to the engine codes of similar models!  Only when these specific set of conditions are met, will the monitor run. Always check your service information for a complete list of the enabling criteria for the specific car you are working on.  Forgetting this step will result in frustration when you don’t understand why a particular monitor won’t run!

A drive cycle is a specific way the vehicle must be driven to satisfy the monitor.  Remember, emissions laws don’t only care about how clean the tailpipe is at idle, but rather care how clean the engine runs during all conditions of operation such as accelerating, decelerating, idle, highway speeds, etc. Sometimes, due to the complexity of the drive cycle, a customer is told to “drive the car for a couple of day” after the repair has been performed so a drive cycle can be completed.  Only then will the car pass emissions inspection. Again, check your service information for the specific drive cycle of the vehicle.

If you’re still a little confused, let me offer this analogy. I teach an advanced diagnostics course. Before a student can enroll in my course, they first need to complete courses in electrical system analysis, engine performance and engine repair. These prerequisites are the “enabling criteria” and must be met before being able to resister for advanced diagnostics.  Once enrolled in my class, the assignments such as labs, tests, quizzes and homework that need to be completed throughout the semester would be my students “drive cycle”! Once they have completed their “drive cycle” for my course, I can then analyze the results and issue a final grade.  

I understand that enabling criteria and drive cycles may not be the most exciting topic when it comes to vehicles, but always remember the devil is in the details. It’s this type of information that will help you to be a more thorough and successful technician. 


Tips From A Tech: So You Want To Buy A Scan Tool?

If the title of this column caught your eye, that means you are most likely interested in purchasing a scan tool of your own. You’ve probably also noticed that the available choices can be overwhelming if you’re not sure what to look for.  Let’s look at some of the features and functionalities that are out there, so you can choose the right tool for the job, without breaking the bank!

How will you use it?

Are you just interested in seeing why the check engine light is on?  Do you need to see live or freeze frame data to aid in your diagnosis?  Or, do you need to be able to interrogate multiple control modules (PCM, ABS, airbag, etc.) with bi-directional support?  If being able to read and clear trouble codes, determine MIL status and check readiness monitors is all that is needed, a basic code reader might be a good option.  However, if you need something more robust to be used as a professional tool, let’s read on!

Generic vs Enhanced OBD2? 

Generic powertrain codes are codes that are common to all vehicles since 1996, and begin with “P0”.  These codes are stored in the computer when an emissions fault is realized by the PCM.  Most of your basic scan tools will easily be able to interpret these codes and give you basic, but useful data.  However, the OBD2 system was designed to allow manufacturers to go “above and beyond” the generic codes.  These manufacturer specific codes begin with “P1”.  To access these codes, a more powerful tool may be needed.  


I would highly recommend finding a tool that can be updated.  My personal scan tool uses my iPhone as the interface, and is updated on a regular basis, free of charge.  Since updates on some scan tools can be expensive, make sure you know the cost (if any) associated with regular updates.  That alone can be a deal breaker!

One-way or Bi-Directional Communication?

One-way communication is the ability to read the information from the PCM without being able to “talk back”.  Look for a scan tool with bi-directional functionality if you will be using it professionally.  Being able to command certain vehicle functions for diagnostic purposes is very important for professional use.  Just recently I had to reprogram the VIN number of a vehicle into the PCM to turn off multiple dash lights!  A code reader or basic tool would not be able to perform that function.  

If nothing else, remember that the best feature of any scan tool is YOU!  Without having the skill to interpret its data, even the most advanced tools will not help you in your diagnosis.  Lastly, don’t get caught up in the features of the tool, but rather determine what YOU need it for.  In my experience, knowing how to interpret a basic scan tool will provide more than enough data for personal use.  The cost of a tool that is capable of bi-directional communication and enhanced data would be much better suited (and justified) for professional use.  


Seeing The Big Picture With AFR Sensors

Today’s modern internal combustion engine is more regulated by state and federal government than ever before. As environmental regulations get tighter, the need for more precise control over tailpipe emissions has become greater.  Modern technology has allowed us to more closely monitor the byproducts of combustion by introducing new components that can more accurately determine fuel delivery at any given moment, resulting in a more efficient and complete burn. Air/fuel ratio (AFR) sensors have largely taken on this role and have become commonplace in today’s automobiles, taking over where traditional O2 sensors have left off.

Are O2 sensors being replaced by AFR sensors because they’re not smart enough?

Not exactly. It’s more like they are being replaced because they are partially “blind.” One major limitation that O2 sensors have is that they were designed to detect only the presence of unburned oxygen, not unburned fuel. It only knows if there is too much or too little oxygen in the exhaust stream. Since it was not designed to detect fuel, it can only “see” half of the air fuel ratio. This is precisely why an O2 sensor is only capable of sending rich, lean or stoichiometric signals to the computer, as opposed to precise air/fuel ratios.

How Do O2 Sensors Work?

Zirconia-based O2 sensors utilize an operating voltage range between 0.100 and 0.900 volts (100mV– 900mV). It is widely recognized that a signal of 450mV reflects a stoichiometric air/fuel ratio of 14.7:1. It is also widely recognized that maintaining a constant 450mV signal from the O2 sensor is impossible due to the rapidly changing conditions to which the engine must adapt (engine load, RPM, temperature, atmospheric conditions, etc.). So, if the computer sees a voltage signal of MORE than 450mV, it recognizes a rich condition, and begins to subtract fuel until it reaches 450mV. If the computer sees a voltage of LESS than 450mV, it recognizes a lean condition, and begins to add fuel until it again reaches 450mV. This is precisely why the upstream (pre catalytic converter) O2 waveform trace looks like a fairly consistent AC sine wave that fluctuates rhythmically between rich (900mV) and lean (100mV). It does this in an attempt to average 450mV, thus achieving stoichiometry.

To better understand the simplistic nature of an O2 sensors operation, I offer this analogy. Imagine having to put a single book in a precise location on a long shelf that contained hundreds of other books while being blindfolded. The only help a friend could offer is to tell you “left” or “right” without expressing how close you were, until you were only one book away from your target location. Once you have arrived at the correct location, your friend would abruptly yell “bullseye!” At no time during the process did you know how far away you were from the target location; you only knew when you had hit your mark. The way you were told “right” and “left” by your friend, is the same way an O2 sensor tells the PCM “rich” or “lean.”

‘Narrowband’ O2 sensors

Traditional zirconia O2 sensors are sometimes referred to as narrowband O2 sensors due to their “narrow” field of view when looking at a vehicles status of either being rich or lean. By design, they report only a rich or lean mixture as it approaches stoichiometry, rather than looking at the entire process of how it got there. For example, imagine watching a NASCAR race by looking only at one “narrow” part of the track. The racecars would enter and exit your view very quickly.  Although you would be able to identify the cars as they pass, you would not understand the context of why or how it happened since you were not able to see the entire racetrack. 

Enter Air Fuel Ratio Sensors

The industry has begun to use AFR sensor because it has the ability to “see” a much wider spectrum for fuel delivery, but, more importantly, can determine a precise air/fuel ratio along that spectrum. Since AFR sensors can control fuel delivery more precisely, they are quickly replacing upstream (pre catalytic converter) O2 sensors in many vehicles. Traditional O2 sensors are still being used as downstream (post catalytic converter) since their purpose is to verify the function of the upstream sensor (and not to determine actual fuel delivery).

A Picture is Worth a Thousand Words

When looking at the wideband analog output (graph 1), you will notice that the output is linear throughout its entire range. Because of this, each point along its output is unique and can be referenced to a specific air/fuel ratio using the mathematical formula (AFR = Voltage x 2 + 10). Try it out for yourself. Pick a voltage on the graph, multiply it by 2, add 10, and you will arrive at a specific air/fuel ratio listed across the bottom. Think of this formula as generating the specific GPS coordinates along the graph for a particular air/fuel ratio!

Graph 1: Notice the narrow operating range represented by the yellow portion of the graph. This narrow range is what limits an O2 sensor from “seeing the big picture.”

I now direct your attention to the narrowband analog output graph (graph 2) which is representative of a traditional zirconia oxygen sensor. Note that while the engine is running rich, the output voltage does not change until it approaches 14.7:1! Since the voltage does not change, a specific air/fuel ratio cannot be determined like the wideband sensor. Only when the ratio approaches stoichiometry does the voltage begin to vary. The same is true as the air/fuel ratio begins to go lean. Beyond roughly 15.2:1, the voltage output does not vary, again not allowing the fuel monitor to determine the precise ratio. As seen on the graph, an oxygen sensor varies its signal only within a “narrow band” of its entire spectrum, and, for that reason, is referred to as a narrowband oxygen sensor.

Graph 2: Note the linear nature of this graph. Since each point along the graph is represented by a unique voltage, the PCM is able to interpret each individual air/fuel ratio. This is the reason it is known as a “wideband” sensor.

How do AFR sensors improve upon traditional O2 sensors?

First, we must understand that traditional O2 sensors change readings by varying their voltage output, while AFR sensors vary very precise amounts of current (amperage). Varying the amperage, instead of voltage, brings both increased precision and complexity.  Since current is being varied on the milliamp scale, it is difficult, if not impossible, to use a DVOM for accurate diagnosis. A scan tool is needed to observe an AFR sensors live data, and, therefore, is the recommended tool for diagnostics

Overall, the design and function of an AFR sensor provides the level of accuracy needed to better comply with current emissions standards.  To further clarify the precise nature of this type of sensor, I’m returning to my NASCAR analogy. Now, rather than seeing a narrow portion of the track like before, pretend that the AFR sensor represents a wide angle lens on a professional camera, equipped with GPS. You are now able to watch the same race, but, this time, not only do you see the track in its entirety, but you also have the precise location of each vehicle as they circle. Since you have a greater perspective, you now have a greater understanding of the race.

The topic of using AFR sensors to determine fuel delivery is enormous. Diagnostic techniques, application, functional differences, computer interpretations are to name just a few variables that must be explored.  Hopefully, you found the information in this article interesting enough to further investigate how the use of AFR sensors help our vehicles remain in compliance with federal and state emission laws. Please feel free to contact me with any questions you may have, or suggestions for further reference material.


Welcome To Fuel Trims!

In practical terms, OBDII is the complex web of sensors, computers and output devices that a modern technician interacts with, via scan tool, to begin the diagnostic process. However, would it surprise you that although it is a convenient interface for a trained technician, its primary purpose is something other than that? In an effort to create the cleanest, most environmentally friendly internal combustion engine possible, OBDII’s primary purpose is to ensure the vehicles engine is operating as emissions friendly as possible. 

The fuel system monitor is a program within the powertrain control module (PCM) that uses various sensors and output devices in an attempt to achieve stoichiometry – an air/fuel ratio that, when achieved, results in complete combustion. This is important because complete combustion is crucial to achieving exhaust emissions that comply with federal law.    

So how does the vehicle know how much fuel to add (or subtract) at any given moment to attempt to achieve such a precise ratio? Through the use of short term and long term fuel trims by the fuel system monitor, fuel delivery is adjusted based on changing variables such as engine load, RPM, temperature and atmospheric pressure, to name just a few.

Understanding fuel trims is the gateway to diagnosing common emissions problems. However, before we begin to use this information, specific criteria must be met in order for the PCM to gather accurate information.

Did We Meet the Enabling Criteria Yet?

Under normal driving and operating conditions, the fuel system monitor must meet a minimum enabling criteria, which once satisfied, allows the monitor to run continuously, assuming the absence of diagnostic trouble codes (DTC’s) and the evaporative emissions purge solenoid is not actively purging hydrocarbons back into the intake. Additionally, the status of the oxygen (O2) sensors must be taken into account as well before proper fuel adjustment can begin to take place. Consult the vehicle service information for additional criteria that must be met.  

A standard O2 sensor sine wave. This represents a properly functioning O2 sensor.

Are We in Closed Loop Yet?

Closed loop refers to the operational status of the O2 sensor(s). If the O2 sensor has not risen to operating temperature, the vehicle is said to be in “open loop” and in turn will use a set of preprogrammed instructions (fuel maps) to determine fuel delivery.  Once the O2 sensor is “live,” the fuel monitor is given the green light to use the O2 signal to adjust fuel delivery.  This is referred to as “closed loop operation.” Only during closed loop operation can short and long term fuel trims be used to diagnose emissions concerns. 

This is short term fuel trim (STFT) at idle in closed loop operation.

Injector Pulse Width…Huh?

I’ve already mentioned that the purpose of the fuel monitor is to add or subtract fuel to achieve a stoichiometric air fuel ratio for emissions compliance.  The only way the engine can vary fuel delivery is through the fuel injectors.

It stands to reason that if the fuel monitor sees a lean condition based on O2 sensor readings, the immediate command would be for the fuel injectors to spray fuel for a longer amount of time. In a similar manner, if the fuel monitor sees a rich condition, the computer will compensate by limiting the amount of time the fuel injector is on. The amount of time that the fuel injector is held open allowing fuel to flow is known as injector pulse width.  The fuel system monitor is in precise control over this.

Great! But What About Those Fuel Trims?

Let’s establish some ground rules! First, when viewing fuel trims on a scan tool, it is viewed as a percentage.  The percentage shown (usually between +25% to -25%) is the amount of fuel being added or subtracted based on current conditions.  This literally means that if the fuel trims are on the positive side, the fuel monitor is adding fuel to compensate for a lean condition. Similarly, if the fuel trims are reading negative, the fuel monitor is reducing fuel to compensate for a rich condition. Think of fuel trims as a window for the technician to see what the fuel injectors are doing.

So What’s the Difference Between Short Term and Long Term?

In practical terms, short term fuel trim (STFT) is the immediate response to a rich or lean condition by the fuel monitor. Long term fuel trim (LTFT) is a learned response based of the activity of the STFT.  STFT is going to immediately react to a situation by adding or deleting enough fuel to correct the condition. The purpose of this immediate reaction is to bring the combustion chemistry back into stoichiometry as soon as possible.  Once enough fuel has been added or deleted to correct the condition (as proven by the O2 readings), the long term will adjust itself to mirror the short term, thus establishing a new zero.

O2 sine wave overlaid onto STFT. Note how each works conversely of each other. When O2 reads lean, PCM commands STFT to increase injector pulse width to deliver more fuel. This is how the Fuel System Monitor attempts to maintain stoichiometry.

This is a Little Confusing…Do You Have an Example?

Imagine you have a vehicle in your garage that seems to be running without issue.  In a perfect world, the STFT would read 0%, LTFT would read 0%, and O2 sensor would be fluctuating rhythmically between 0.1v-0.9v.  While its running, you accidentally disconnect a vacuum line, creating a small vacuum leak.  Because air is now entering the engine at the point where the vacuum line was disconnected, the O2 sensor sees this and reports a lean condition. The fuel monitor sees this lean condition, and immediately tells the STFT to begin to add fuel until the O2 sensor begins to read normal again.  So, as an example, let’s say the STFT begins to add fuel until it reaches +13%, when at that point, the O2 signal returns to normal. Since the O2 is now reading normal, the STFT no longer needs to increase fuel delivery, and begins to level off (at +13%).

At this point, LTFT has been looking at what the STFT has done, and now that short term has leveled off, LTFT begins to increase to +13%.  Once long term reaches +13%, it now levels off, and the fuel monitor establishes +13% as the new baseline.  Since this is the new baseline and everything has been compensated for, STFT does not have to make any further corrections, so it begins to fall back to 0% (remember, STFT is only there to make immediate corrections, not solve the problem.  Once LTFT took over, STFT was no longer needed).  At this point, if a technician were to take fuel trim readings from the scan tool, they would read STFT 0%, LTFT +13%, O2 .1v-.9v.

Suddenly, you realize what you had done, and reconnect the vacuum line. At this point you have stopped the leak, so the engine is back to receiving the amount of air it originally needed to maintain stoichiometry. However, the LTFT is still adding +13% more fuel, so now you have just created a rich condition!  Well, the O2 sensor sees this rich condition, and immediately reports this to the fuel monitor. The fuel monitors response is (you guessed it!), to tell STFT to immediately reduce the amount of fuel by decreasing the injector pulse width.  So now the STFT begins to decrease fuel, and eventually gets to (you guessed it again!) -13%.Once it reaches -13%, the O2 sensor again starts to read normal, and STFT begins to level out at -13%. Now, LTFT (which at this point is still at +13%, because it still hasn’t moved from when it was compensating from the original vacuum leak), sees that since STFT has leveled out after reducing fuel by 13%, so it too will do the same and reduce itself by 13%, bringing it back to 0%.  At this point, the problem has been solved and the fuel monitor is back to its original readings.


I hope you enjoyed this very brief introduction to fuel delivery.  Understanding fuel trims, O2 readings and fuel monitor operation goes far beyond what can be learned in one article. Engine RPM, DTC’s, and MIL status are just a few other factors involved in using fuel trims as a diagnostic tool. However, building a proper foundation is the first step. My intention as an instructor (and technician) is for this article to build the confidence and interest to continue to explore this subject. Please feel free to contact me with any questions you may have or advice on some great resources.