Damaged Hard Parts Create Erratic OSS Output – 10R80

This case study highlights the diagnosis and repair of a 2019 Ford F-150 with a 10R80 transmission issue initially flagged by code P0721. Although the symptoms suggested a faulty output speed sensor, further investigation revealed deeper mechanical problems. Through persistence, waveform analysis, bench testing, and collaboration with the warranty company, the root cause was traced to pitting in the output carrier that corrupted the OSS signal. After replacing the transmission with a remanufactured unit and performing the necessary reprogramming, the truck returned to normal operation. The experience underscores the importance of thorough diagnostics, combining scope work, research, and mechanical inspection, and demonstrates how real-world persistence can prevent misdiagnosis and ensure long-term repair success.

This spring, our shop had the opportunity to diagnose and repair a 2019 Ford F-150 (3.5L EcoBoost, 10R80) with a transmission issue that turned out to be more than just a sensor glitch; it was a lesson in persistence, pattern recognition, and the value of bench testing.

The customer reported a stored transmission code P0721 (Output Shaft Speed Sensor Circuit Range/Performance) and described a troubling behavior: from a stop, the truck would occasionally feel like it was in neutral. The issue was becoming more consistent. They had previously visited a local Ford dealership, but because their extended warranty was aftermarket and not honored by the dealer, they came to us for help.

After gathering information from both vehicle owners, we began by scanning the TCM. Sure enough, P0721 was present, along with P061A (Internal Control Module Torque Performance) and P0402 (Invalid Data Received from TCM) stored in the ECM.

On the initial road test, the transmission would jerk from a stop and enter limp mode. While monitoring scan data, we noticed the output speed signal intermittently dropped out during these events. (Figure 1)

Figure 1

Fluid level and condition were good: red, no burnt smell, no leaks. The external harness looked untouched. Battery and alternator ripple tests passed.

At this point, we suspected a fault with the output speed sensor (OSS) or possibly the internal harness.

Because the warranty didn't cover electrical components, we obtained authorization from the customer to proceed. Before diving in, I reviewed ATSG's 10R80 documentation, watched a helpful video from SIU Automotive, and studied electrical diagrams and connector views. With a better understanding of the system, I felt confident moving forward.

The connector and terminals looked clean, no corrosion or tampering. Next, I scoped all four speed sensors: intermediate, turbine, and output. Voltage to the sensors was verified at 9 volts.

While scoping the output speed sensor, I noticed something unusual. These sensors don't just measure rotational speed; they also detect direction. The waveform pattern changes depending on whether the rotation is clockwise (drive) or counterclockwise (reverse).

In drive, the OSS signal intermittently showed large, wide spikes, inconsistent with the expected narrow pulse pattern. This suggested the sensor and TCM were functioning correctly, but something was interfering with the signal, possibly a physical defect in the output carrier. (Figure 2)

Figure 2

We contacted the warranty company and recommended transmission replacement. Unfortunately, they wanted more proof. They authorized replacement of the OSS and a pan inspection.

Thankfully, valve body removal on the 10R80 isn't overly complex. The pan showed no notable debris. I removed the auxiliary fluid tube and seal, unlocked the internal harness, and removed the valve body bolts, leaving the center bolt (#1 in the diagram) for alignment. (Figure 3)

Figure 3

With the valve body out, I removed the auxiliary stop/start pump and the output speed sensor. I also removed the rear driveshaft to manually rotate the output carrier and inspect for damage. No obvious issues stood out. (Figure 4)

Figure 4

Back at the bench, I tested the old harness with a halogen lamp to check current capacity. It passed, even with wiggle testing. Still, a pin fitment issue or faulty sensor could cause dropouts. I began to wonder: Was it just a bad sensor after all?

After replacing the OSS, the truck's symptoms persisted. We convinced the warranty company to send an inspector. After reviewing our findings and walking through the diagnostic process, the inspector agreed: the issue was likely with the output carrier.

We installed a remanufactured unit, performed reprogramming, solenoid strategy, ID setup, and final drive procedures. No more OSS codes. No more limp mode. The truck was back to normal, and the customer was thrilled.

Curiosity got the better of me. I wanted to reproduce the OSS signal issue on the bench. Using a method from the SIU Automotive video, I built a test harness powered by a 9V battery, using 220-ohm resistors, wire, solder, and flux.

With the valve body removed again, I scoped the OSS signal. Just like in the vehicle, the drive signal showed intermittent large spikes. Reverse signals were consistent.

I removed the auxiliary pump to get a closer look at the windows on the output carrier. That's when I found it: pitting in two of the sixty windows. The pitting was on one side, which explained why reverse signals were unaffected; reverse rotation didn't pass over the damaged windows in the same way. (Figure 5)

Figure 1

This case was a reminder that not every transmission issue is solved by replacing a sensor or clearing a code. Sometimes, the fault lies deeper: in the mechanical interface between electronics and hardware. The OSS was doing its job. The TCM was interpreting the signal correctly, but the carrier itself was sending corrupted data.

Without bench testing and a willingness to go beyond the basics, this issue might have been misdiagnosed. And without the right tools and knowledge, it might have been dismissed as a software glitch.

In the end, it was a combination of scope work, research, and persistence that led to the correct diagnosis. While the customer got their truck back in perfect working order, I got something too: a deeper understanding of the 10R80 and a renewed appreciation for the detective work that transmission diagnostics can be.