Using the Auto-Scanner (scan tool) and PID (parameter) Interpretation

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Procedures to use and get the most of your OBD-II scanner

OBD-2 fuel trims diagram By Mandy Concepcion

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Fig-Fuel trim scale showing the entire FT monitor range. Notice the OBD II scale at the top and the older GM scale at the bottom. The equivalent Stoichiometric value of OBD II to GM’s is 0.00% = 128.

In recent years the scan tool, as it became faster and more powerful, has become the equipment of choice for many technicians. It is by far the first tool employed at the start of the diagnostics process and with good reason. The scanner is versatile, with many built in features that no other piece of equipment can match. It is also the only tool that can provide a window into the ECM inner operation and memory functions. In essence, it tells you what the ECM is seeing, regardless of whether it is true or not. With the scan tool, a whole array of convenient and fast techniques can be employed to quickly analyze and diagnose a particular problem. As time goes on, the scanner will see an even broader range of operations, since it is bound to become much faster with newer advances in electronics. It may even get to rival the fastest equipments, like the oscilloscope, at some point-in-time in the future.

This section deals with the proper use of the versatile scan tool. Many different diagnostics techniques will be introduced, as well as ways to make the most out of your scan tool. In the last couple of years the term “PID diagnostic” has been used to denote the ability to diagnose a problem by analyzing the serial data on a scan tool. In fact, given today’s faster scanners, it is possible to perform a great deal of the diagnostics process sitting inside the vehicle. This gives rise to the term “front seat diagnostics”. As much as 70% of the diagnostic work can be perform by the simple correlation of signal data, with the rest employing actual manual testing. As of right now, and probably not for a long while, the scan tool will not replace the trusty VOM or the scope, but its proper use will make things a lot easier due to the time savings. All that translates to money in your pocket.

Serial data communications has been around for a number of years. As far back as the early 1980’s, domestic manufacturers were putting out vehicles with available scan tool data parameters. In the early days, the communication protocols were proprietary in nature, which made it harder for aftermarket equipment makers to come up with affordable scanners for the average technician. Although the need for an OEM scan tool is as important now as ever, a wide range of engine performance and emission faults can be quickly diagnosed by the use of “generic PIDs”. In spite of the fact that the OBD II generic PID serial data stream is many times thought of as being slow and void of any diagnostic importance, this is definitely not so. The generic OBD II protocol works on a request system, which means that the scan tool has to actually ask the ECM for each PID (In OBD II a PID stands for parameter identification). This OBD II request operation contrasts with the OEM communications protocols that work using data packets, whereby, the data stream PIDs is sent in bursts or packets. However, generic OBD II standardized the whole communications process and made it possible to, at least, have access to a minimum of data for diagnostics regardless of make and model. In generic OBD II, by simply reducing the amount of PIDs on the screen a faster data rate can be obtained, since the scanner has to request less data. By combining different and faster data PIDs to form a relationship, a signal correlation can be arrived at. An example is an EGR valve that is commanded on (manually or otherwise) which should have an effect on the MAP sensor. A lack of MAP sensor response is a good indication of a defective or clogged EGR valve, since an opening EGR valve should create a drop in intake vacuum. The same PID strategy is also employed by the ECM when running each drive cycle. The difference is that we can also use these techniques to our advantage when diagnosing a vehicle.

This section will have a somewhat different approach to the rest of the book. An effort has been made to organize the section by the particular problem found and how to diagnose such faults using the different PIDs available to the manufacturer in question. Both OEM (enhanced) and generic information formats (PID data signals) will be implemented in the diagnostics strategies presented here. It is also worth mentioning that in order to attain maximum advantage of these techniques, the use of a graphing-software is highly recommended. If your scan tool has a graphing feature, use it, since it will make the whole process a lot easier. The mind can process graphical representations much faster than just raw numbers. This section has been arranged by using PC generated PID graphing for ease of publishing. Provided that your scanner has a graphing option, the principle is the same. Enjoy the rest of the section.


By far, fuel delivery problems account for as much as 60% of all CEL (check engine light) complaints. Modern OBD II (generic and enhanced) as well as late ODB I diagnostic systems offer extensive opportunities for fault detection. These are the parameters that determine fuel delivery, in order of priority.

Base injector pulse-width, as determined by the base fuel-injection-map.

Engine coolant temperature sensor.

Engine RPM.

MAP (speed-density) or MAF sensor.

TPS (throttle angle/position).

O2 sensor.

IAT (intake air temperature sensor).

Battery voltage.

External Loads placed on the engine.

The following are the PID associated with fuel related problems.

IAT (intake air temperature) sensor is a secondary analog input to the ECM. Is tells the ECM the temperature of the incoming air. On engines with the IAT screwed into the intake manifold, it is called an air charge sensor. A high air charge sensor temperature reading is a good indication of a clogged exhaust. The reason being is that the backed-up exhaust gasses accumulate on the intake manifold, causing the high air charge sensor temperature reading.

BARO (barometric sensor) is either an analog input or a calculated value to the ECM and can also be considered a main input to the ECM. On vehicles with a separate BARO sensor, the measurement is taken directly. However, the vast majority of fuel systems use the MAP or the MAF sensor for barometric pressure calculations. In MAP systems (speed/density), the BARO value is arrived at by measuring the MAP sensor at either W.O.T. or KOEO. In either case manifold vacuum is nonexistent and the actual reading indicate atmospheric pressure.

ECT (engine coolant temperature) sensor is an analog input to the ECM. This sensor is a main ECM input, and sets the base injection and ignition characteristics. The ECT sensor also tells the ECM when to go to closed-loop, as soon as a pre selected warm-up temperature has been reached.

ENGINE LOAD is a calculated PID. The ECM takes the RPM, TPS and the MAP/MAF into consideration when calculating the engine load. Some manufacturers (OBD II) report this parameter with a negative load factor included. In other words, if a vehicle is traveling down hill its momentum would be driving the engine, creating a negative load condition. This value would be factored into the overall load PID value, therefore, the normal load values would always be higher than for other manufacturers.

The RPM is a calculated value arrived at from a CRK sensor or the ignition module/pick-up coil input to the ECM. The RPM is a main input and should always be considered when analyzing any PID group.

FUEL TRIMS is a calculated PID and is usually expressed in percentage. The fuel trim is the calculated value of the adjustments performed by the ECM to the base injector pulse. The fuel trim PID is always divided into LTFT (long term fuel trims) and STFT (short term fuel trims). The LTFT are the long-term adjustments performed by the ECM to the base injector pulse. This parameter is an indication of the ECM response to more persistent and influential A/F ratio faults, such as a large vacuum leak (lean) or a punctured fuel pressure regulator (rich). Also, the LTFT only changes value after the STFT has reached its maximum limit and the ECM can no longer adjust the mixture. The LTFT can be thought of as a slow acting parameter that only intervenes when its partner, the STFT, can no longer correct the mixture. The STFT are the short-term adjustments performed by the ECM to the base injector pulse. This PID reacts very fast to changes in the A/F ratio. The ECM will always try and keep the STFT as close to 0.00 % as possible. So long as it can maintain a stoichiometric A/F ratio through smaller corrections to the injector pulse, the STFT will hover at a maximum value of + or – 8 %. In the event that a greater A/F ratio fault exists, the ECM will not be able to correct the problem through smaller corrections and the LTFT will increase, thereby, taking the STFT back to close to 0.00 % again.

OBD-II PID parameter chart

Fig – FUEL TRIM chart depicting the relation between FUEL TRIMS, O2 sensor and RPM. Notice how the ECM tries to keep the STFT close to 0 %. This vehicle was misfiring due to a defective ignition coil. Ignition as well as injector (not pulsing) faults render the exhaust gases with excess O2. The O2 sensor will perceive this as a lean condition, even though there is excess raw fuel being put out by the misfiring cylinders. The O2 sensor is only concerned with the Oxygen content of the exhaust.

About 60 % of all CEL (check eng. light) faults are A/F related. General Motors was the first manufacturer to put out a fuel trim PID. As far back as 1981, GM vehicles were using fuel trim values, which they called “block learn – LTFT and integrator – STFT”. Older GM vehicles use the lower scale on the chart, which puts 14.7:1 A/F ratio at 128 (OBD II at 0.00%). This older fuel trim scale can still be found today as a scanner PID, thereby, complimenting the newer OBD II generic scale.

Example 1 – An engine (V-6) is operating, at idle, within normal specifications. The STFT are at 3 % (normal) and the LTFT at 2 % (normal). Suddenly an injector goes faulty and starts dripping 20 % more fuel (rich condition). This can be considered a minor fault. At this point in time the O2 sensor goes high (rich) and the ECM compensates by decreasing injector pulse, thereby, sending the STFT to – 5 % or so. This A/F ratio fault can clearly be controlled by the ECM through minor corrections and will probably not change the LTFT values.

Example 2 – The same engine is again operating normally, with the STFT and LTFT values within proper range. Suddenly the intake manifold gasket ruptures and a large vacuum leak is created. At this time, the O2 sensor voltage goes low (lean) causing the ECM to increase injector pulse time to try and correct the fault. This action raises the STFT to its maximum of 25 %. After a short time, the ECM samples the O2 sensor and still sees a low voltage. The ECM then increases or adds more on-time to the injector pulse, again trying to correct for the lean condition. This action will raise the LTFT from 2 % (normal) to 10 % for the first time. This cycle of A/F correction will continue until the mixture is brought back to stoichiometry and the O2 sensor starts switching again, in which case the LTFT would stay high and the STFT would go back close to 0 %. In the event that the vacuum leak becomes too large, the STFT as well as the LTFT would reach their maximum positive values and the ECM will set a faulty code for a lean condition. At this point, the STFT and LTFT would be at around 25 % to 35 % or maximum, depending on the manufacturer. The one important fact about fuel trims to remember is that even if they may be off, the A/F ratio is still at stoichiometry or 14.7:1. The fact that the fuel trim values are off only means that the system is operating outside the base injection map, by the same factor. The following graph shows the fuel trims in action.

As a last note regarding fuel trims, this parameter re-zeroes under different load conditions. Every time the ECM switches to a different cell, the fuel trims re-zeroes and relearns a different adaptive value.

 NOTE: This training blog is taken from our book "Diagnostic Strategies of Modern Automotive Systems". For further -how to test- instructions visit our book section on our website

Copyright © Mandy Concepcion, Automotive Diagnostics and Publishing