Written By, Mike McGlothlin
It’s a part that can fail on any modern engine and also one that can be found in most vehicles built over the course of the last 25-40 years. It’s called a throttle position sensor, and its failure to perform as intended can lead to distinct drivability and performance issues—and it can even leave you stranded. In the case of the Cummins ISB 24-valve 5.9L, an inline-six diesel power plant we’re very familiar with here at Big Bear Engine Company, it’s an infamous weak link. Known in this application as the Accelerator Pedal Position Sensor (or APPS), it’s been one of the most notorious issues associated with ’98.5-’02 Dodge Rams for more than two decades.
So, what causes the APPS to fail? And why does it fail? Below, we’ll explore the “what,” “why,” and “how” behind this frequent failure point—along with sharing the common diagnostic trouble codes it sets and what it’s sometimes misdiagnosed as. Additionally, we’ll show you how to reset, recalibrate, and even clean your ailing APPS in an effort to restore its operation before you have to spend any money. But should you have to purchase a new APPS, we’ve got you covered there, too. We’ll clue you in as to which ones to buy and which units to stay away from. Get ready to make your 24-valve Cummins’ intermittent dead-pedal issues a thing of the past.
What Is The APPS?
The Accelerator Pedal Position Sensor (or APPS) employed on the Cummins ISB 24-valve
5.9L is essentially a throttle position sensor with a fancy name. It is part of the bell crank assembly on these Cummins engines (specifically located on the other side of the bell crank shown here) and is an early version of fly-by-wire, where a cable spans from the accelerator pedal to the bell crank and sensor. The APPS provides the truck’s ECM with a DC voltage signal, which varies as the position of the accelerator changes. An APPS on an ISB Cummins-powered ’98.5-’02 Ram typically operates within a range of 0.5 (idle) and 3.9 volts (wide open throttle).
Why The APPS Fails
Causes behind why an APPS fails on the ISB Cummins run the gamut. Age, vibration,
prolonged exposure to heat, and Father Time can all be the death of them. However, faulty replacement APPS’s—usually cheap aftermarket knock-offs—account for many headaches as well (Pro Tip: stay away from the $30 Amazon and eBay sensors). Of course, an improperly calibrated replacement APPS can also be a big problem, and one that causes poor drivability, either intermittently or on a consistent basis. No matter what the exact cause of an APPS failure is, a complete loss in throttle response can present a pretty scary scenario—especially when you consider that most of these workhorse engines are used to tow heavy loads, day-in and day-out.
How The APPS Fails
During the early stages of an APPS failure, drivability issues become more and more noticeable. Erratic engine behavior such as surging, bucking, and high idling are all common and occur due to a gradual loss in contact between the APPS’s internal rotating fingers and the resistance track they ride in. The result of the APPS’s electric signal coming and going in such a rapid manner leads to unpredictable engine performance. Common diagnostic trouble codes (DTCs) thrown when an APPS is checking out include P0122 (for low voltage) and P0123 (for high voltage), but P0121, P0120, P0124, P0125, P0222, P0223, and P2121 can all be set as well.
The #1 Sign Your APPS Is Checking Out: Dead Pedal
It doesn’t get much more helpless behind the wheel than during a dead pedal incident. The
most common symptom of a dead or dying APPS is dead pedal, and it occurs when the aforementioned internal rotating fingers lose all contact with the resistance track they operate in within the APPS assembly. During a dead pedal situation, the engine will idle but be completely unresponsive to throttle input. You’ll also typically get a CEL, with at least one of them being a DTC listed above. It’s important to note here that some dead pedal (i.e. failing APPS) conditions are misdiagnosed as a bad VP44 injection pump. Codes associated with the latter failure typically yield a P0215, P0216, P1688, and/or P1689.
The #2 Sign Of APPS Failure: Random Drivability Issues
Sporadic, random, and radical drivability issues are the best way to describe the effects of
a failing APPS. Engine speed can (and often does) surge, buck, or cut out at times, and idle issues can also occur. Additional bad throttle postion sensor symptoms include sluggish throttle response, a seat-of-the-pants lack of power, and shifting issues in a truck equipped with an automatic transmission. In particular, the ISB Cummins-powered ’98.5-’02 Dodge Rams equipped with the Chrysler 47RE four-speed automatic can experience a poor 1-2 upshift or a torque converter that rapidly unlocks and then relocks in third and fourth gear.
Potential Remedy #1: Resetting The APPS
Without getting your hands dirty, you can follow a simple process of resetting the sensor to see if your drivability problems subside. Resetting the APPS forces the truck’s computer to relearn where the throttle position is at all times. Start by disconnecting the batteries, turn the ignition to the ON position, and then walk away for 15 minutes. Next, turn off the ignition, reconnect the batteries, and turn the key on again without starting the engine. Now, slowly (and progressively) push the accelerator to the floor and then slowly (progressively) release it, followed by turning the key to the OFF position once more. You can then cycle the key on and start the engine, the APPS having been reset.
Potential Remedy #2: Recalibrating The APPS
Going beyond resetting the APPS, this procedure calls for a few simple hand tools and a
voltmeter. With the key on but the engine off, the APPS wire is probed in order to find its voltage output (which should be somewhere between 0.5 and 0.6 volts), and the number is remembered (or written down) for later reference. Then, with the ignition turned back off, the black plastic APPS cover is removed, followed by the six 10mm bolts that hold the APPS bracket in place (note: do not remove the cables). The APPS’s T-20 torx bit screws (shown) should then be carefully loosened, and the voltage number present on the APPS tag compared to the voltage you obtained earlier. If the two don’t match, the APPS requires adjustment, which is done by rotating the APPS clockwise and/or counterclockwise.
Potential Remedy #3: Cleaning The Contact Points
It’s not easy to do, and in fact should only really be performed as a last resort, but spraying
contact cleaner inside the APPS assembly’s internal resistance track can restore proper functionality. This is especially true if dirt, dust, grime, or other debris has caused the rotating fingers to lose steady contact with the resistance track. This potential fix doesn’t require you to pull the APPS off its mounting bracket, which means there is no need to adjust or calibrate the APPS, but it does call for a small, carefully drilled hole to be made in the APPS assembly in order to accommodate a spray can straw. Full disclaimer: if you’re venturing this far into a failing APPS, you’re likely better off just purchasing and installing a new sensor.
The Ultimate Fix: A New APPS
Of course, the most logical solution to intermittent or outright APPS failure is to just replace the entire sensor. However, be leery of cheap replacement sensors available online. These low-buck knockoffs often fail to hold calibration with the truck’s computer like the OEM version does, meaning drivability issues can return in short order. Instead, look for a known-reliable replacement APPS from a reputable supplier. The TIMBO aftermarket APPS (shown) is a quality piece found at Geno’sGarage.com as well as other online retailers and allows you to reuse your original bell crank plate rather than replace it. It retails for roughly $200, whereas an OEM Cummins APPS assembly can run you more than $400.
When You Fix It Right…
To be clear, an APPS replacement is never simply a bolt-on and go solution. A new APPS
will have to be calibrated, following the same sequence detailed in Potential Remedy #2 above. If you fail to calibrate the new APPS (or do so improperly), the computer won’t precisely know where the idle to off-idle/driving position is, and you will have drivability issues. A quality replacement APPS for the Cummins ISB 24-valve should last a minimum of 10 years—and everyone familiar with these old 5.9L’s knows you can get a lot of work done in that amount of time.
