The APP, but what is an APP?

The APP sensor is the modern equivalent to the accelerator cable. Most vehicles today have no accelerator pedal. The reason behind this seemingly useless technology is to match the engine power output to the wants and needed of the driver. Older throttle cable systems lag behind the required power requirements, with the end result being excessive emissions and loss of fuel economy.

Theory of Operation

Modern automotive safety systems rely heavily on electronics to make it all work. The newer accelerator system, without any cable linkage, is no exception. This system is generically called drive-by-wire and it acquires different names depending on the manufacturer. Drive-by-wire integrates an electronically controlled throttle plate with a computer. Features such as cruise control, automatic idle adjustment, and other safety features such as automatic deceleration if the vehicle goes out of control are simply added as software add-ons. This makes it very inexpensive for manufacturers to keep up with newer technologies.

The accelerator pedal position sensor (APP) is of central importance to the drive-by-wire system. This sensor is designed to provide the ECM with accelerator pedal position as well as its rate of change or how fast the driver is pushing on the gas pedal. Besides providing the ECM with accelerator position, the APP is also associated with other modes of operation.

One such mode is the reverse mode in which the ECM changes the acceleration pattern of the vehicle when backing-up and the selective cylinder-kill function if part of the APP sensor fails. In this last case, the ECM cuts injector pulsation every other engine revolution in the event of an APP failure. This reduces performance, but allows the driver to reach a repair shop.

The APP sensor is basically a potentiometer or variable resistor with a second potentiometer acting as a redundant sensor in case of a failure (some systems also use a third potentiometer). In other words, the APP is two or three position sensors in one. The first sensor (sensor 1) is the main input to the ECM for throttle plate control and the second sensor (sensor 2 or 3) is for redundancy. The ECM is constantly comparing all potentiometer readings against each other. In the event of a discrepancy the ECM sets a code and goes into a reduced performance mode (limp home mode). The ECM also provides a separate reference voltage and ground to each potentiometer, regardless of whether 2 or 3 pots of are used. If the reference voltage or ground is lost to one of the potentiometers the others are still able to function. This allows the driver to drive the vehicle, with limited performance, to the nearest shop. These series of potentiometers within the APP sensor are also wired differently for the sake of redundancy.


Understanding the system’s wiring pattern is very important for correct diagnosis. A quick look at the wiring diagram will reveal the particulars to the system being worked on. On certain systems, it is common to see one of the APP potentiometer signal increase (low to high) when pressing on the accelerator, while at the same time the other potentiometer signal decreases (high to low).

The APP signal characteristics is dependent on how the APP sensor is wired. If the potentiometer’s wiper rests (throttle closed) at ground, then the signal output increases as the throttle is pressed. And if the wiper rests at the 5 volt reference, then the signal decreases as the throttle is pressed. The important thing to remember is that regardless of how many potentiometers are within the APP, they all have separate reference voltage and grounds lines.

A throttle control actuator is also employed in drive-by-wire systems to do the actual throttle opening. The ECM responds to the APP signal changes by opening the throttle plate. Some systems also use a separate throttle control computer or throttle control module.

The throttle control module (if separate) will always work in conjunction with the engine control module in order to control the vehicle’s acceleration. Both modules communicate with each other via a serial data line. In older systems there were multiple sensor connections from and to each of these modules. But newer and faster ECMs have made it possible to rely on data line communications and not hard-wired systems. As the APP signal changes, the ECM commands the throttle actuator motor to open the throttle plates a certain amount. The throttle actuator assembly TPS sensors then relay the throttle opening back to the module. The APP signal is constantly being compared to the throttle actuator assembly

NOTE: It is important to know that under no circumstances should the throttle plates be forced open with a screwdriver. Some techs have tried this using screwdrivers or pliers to accelerate the engine, as with older systems (non drive-by-wire). This action will cause a discrepancy between the APP signal and the throttle actuator TPS sensors causing the vehicle to go into limp-in-mode (loss of performance). In most cases, the use of a dedicated OEM scan tool is needed to reset this condition and re-learn a new throttle actuator position adaptive memory.

position sensors. Any discrepancy between the APP and the TPS sensors will set a code, with the system also going into limp-in-mode. The ECM flags a discrepancy when the deviation or difference between the APP and the TPS signal goes above a maximum pre-programmed amount. All drive-by-wire systems use dual TPS sensors. Again this is all done for redundancy. 

In drive-by-wire systems, idle speed is achieved in two ways. Some systems use a regular IAC valve to control idle speed. There is no basic difference between the old and this new IAC valve. The only exception is in the internal ECM programming, which takes into account the differences in the throttle actuator.

The second way to control idle speed is by the ECM using the actual throttle actuator to slightly open the throttle plates. This action, in conjunction with the engine speed input, and the TPS signal determines the idle speed. As a side note, it should be pointed out that in older (non drive-by-wire) systems the throttle plate and bore carbon deposits were the cause of a good deal of idle air/fuel mixture problems. However, in drive-by-wire systems, dirt and carbon deposits could render the engine inoperative by making it unable to idle properly. It all goes back to the fact that any discrepancy between the APP and the TPS will cause a variety of different problems. In this example of an engine with a dirty throttle body, the carbon and dirt creates an air restriction causing the ECM to increase or change the throttle plate opening. The ECM has pre-programmed in its internal memory the maximum throttle opening at idle possible. If this limit is reached, (because of the carbonized throttle body) a trouble code is set in memory and the system goes into limp-in-mode. A throttle body cleaning will correct any of these problems.

Conditions that Affect Operation

As stated before, dirt and carbon deposits in the throttle plates have a negative effect on throttle control. The ECM will always try to adjust or compensate for the air restriction in the throttle body. This excessive throttle opening creates the APP to TPS signal discrepancy, triggering the ECM into setting a code.

The APP dual or triple potentiometer signal must also be within proper specifications. An improperly adjusted (if adjustment is available) APP sensor at the rest position will be picked up by the ECM, as is a bad or erroneous signal. Also, the two or three APP signals must be without discrepancies within each other. If the main signal fails the ECM then looks at the other signals for proper accelerator pedal position. This will set a code and performance will be reduced.

The throttle actuator is an actual electric motor (stepper or DC motor) that is energized by the ECM or throttle module in response to the APP signal. The actuator has a set of gears and springs which enables it to work the throttle plates. Any binding of the gears or breakage of the springs will create an immediate discrepancy between the APP and the throttle TPS signals. The same applies to a defective throttle actuator motor itself. Again, a discrepancy beyond the preprogrammed acceptable limit will set a code and the ECM will go into reduced engine performance. On certain occasions, if the TPS dual signal is completely lost the engine will shut down. Hence the use of two TPS signals with independent voltage reference and ground. Again, all done for redundancy.

Component Testing

An invaluable tool in diagnosing APP and drive-by-wire system problems in general is the scanner. An after market or preferably an OEM scan tool analysis, with all the parameters (PIDs) necessary for proper signal monitoring makes for a quick preliminary testing of the system. A quick analysis of the APP PIDs will indicate if the potentiometer signals are out of specifications and any possible signal discrepancy can be picked up by analyzing the different PIDs. A careful observation of the APP signals, while slowly pressing on the accelerator cable, will reveal a faulty APP sensor (provided that the scan tool is fast enough, as with some OEM scanners) . Otherwise, a multi-channel VOM or scope should be used. It is also worth knowing that most systems will put out throttle position ERROR PID. Watch carefully for this parameter, since an error flag will reveal an APP to TPS discrepancy.

NOTE: If an OEM scan tool is available and the manufacturer has bi-directional control of the throttle actuator, perform an increasing throttle opening command and observe the TPS output on both the scope and the scanner PIDs. Always be on the alert for any APP to TPS discrepancy that might send the system into limp-in-mode.

Despite today’s faster and better scan tools, the second step to this procedure should always be followed by a manual electrical check.

• The first step is to prove the APP voltage reference and ground circuits. These circuit are provided by the ECM independently of each other, for redundancy.

Proceed to the throttle computer (look up the proper name according to the manufacturer) and uncover the wiring at the connector itself.

• Check the signal voltage output by using a multi-channel VOM or oscilloscope. Connect each channel to the two or three potentiometer output signals. The need for a multi-channel scope is becoming more apparent as more sensors will be added to future vehicles.

• With the scope connected, check the output signals with the APP sensor at rest. Compare to proper specifications. Then slowly press on the accelerator pedal and observe for any glitches or sudden drops in signal voltage. This procedure is somewhat similar to checking a TPS sensor. The difference is that it is a dual or a triple TPS (depending on manufacturer).

• Notice the correlation of all the signals to each other. If one of the potentiometer signals output is off calibration, the ECM will set a faulty code.

If the APP sensor passes the test, disconnect the scope/VOM from the two redundant potentiometers. Then, connect the second and third channels to the dual TPS at the throttle plates. Leave the first channel connected to the primary potentiometer. With the engine running accelerate the vehicle slowly. Observe the TPS outputs for a smooth increasing signal, as during a normal TPS check. This step will indicate any throttle actuator binding or TPS problems. The test will also uncover a possible ECM transistor driver problem or an electrically faulty throttle actuator motor.

NOTE: If an OEM scan tool is available and the manufacturer has bi-directional control of the throttle actuator, perform an increasing throttle opening command and observe the TPS output on both the scope and the scanner PIDs. Always be on the alert for any APP to TPS discrepancy that might send the system into limp-in-mode.

• A couple of other scan tool data PIDs are also helpful in diagnosing this system. If the system uses the throttle actuator for idle control, look for an out of balance air/fuel ratio mixture. The long term and short term fuel trims (LTFT & STFT) are helpful PIDs when it comes to diagnosing air/fuel ratio problems. Provided that the vehicle has no vacuum leaks or fuel restrictions, an out of adjustment or faulty TPS will send the wrong signal reading to the ECM. A maladjusted TPS could make the ECM react as if the throttle plates were in a different position than they really are.

The net effects will be the ECM increasing throttle plate opening (this will create a lean condition), or reducing throttle opening (this action will create a rich condition or a possible engine stall).

By studying the system PIDs carefully before jumping into a long diagnostic routine, it is possible to pin point the fault faster and easier. Knowing the system is the first rule to remember in modern vehicle diagnostics.


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