Experiencing strange transmission behavior in your car can be unsettling. For Acura TSX owners, like myself, encountering issues like rough shifting or hesitation can lead to immediate concern. Recently, I faced a perplexing situation with my 2004 Acura TSX where the transmission seemed to be acting up. This led me down a path of DIY discovery, and ultimately, a successful fix that I’m eager to share, especially for those who might be searching for solutions beyond just relying on OBD2 scanner apps.
It began subtly. My 5AT TSX, with around 99,000 miles, started exhibiting slightly harder shifts and a tendency to shift gears unnecessarily often at lower speeds. To better understand what was happening, I switched to sportshift mode. That’s when things escalated quickly. Pulling away from a stop, a loud clunk reverberated through the car, and the dreaded check engine light illuminated, accompanied by the VSA and triangle warning lights. The sportshift mode became unresponsive, gear indicators vanished, and the car struggled to accelerate, feeling severely underpowered. My initial fear was a catastrophic transmission failure.
After letting the car cool down, I cautiously drove it around the block. The check engine light remained on, but the other warning lights had disappeared, and the gear shifting seemed to return to normal. To get to the root of the problem, I turned to a common diagnostic tool – an OBD2 code scanner, borrowed from a neighbor. While there are numerous apps available that can connect to OBD2 scanners for diagnostics, sometimes a dedicated tool is readily at hand. The scanner revealed a P2138 code: “Accelerator Pedal Position (APP) Sensor 1/2 (Throttle Position Sensor D/E) Incorrect Voltage Correlation.”
Extensive online research shed light on the TSX’s drive-by-wire system. It involves a cable connecting the accelerator pedal to an APP sensor module located in the engine compartment. This module interprets pedal position and relays it to the ECU. It’s believed that the module contains dual sensors, sending both readings to the ECU. A P2138 error arises when the ECU detects a significant discrepancy between these sensor readings. This triggers a “gimp mode,” limiting engine RPM and power to protect the system. Restarting the engine can temporarily clear the error and restore normal operation.
For about a week, I continued driving, paying close attention to the accelerator pedal. Retrospectively, I realized a gradual decline in pedal smoothness and responsiveness at lower ranges had been occurring. Smooth starts became challenging, and maintaining a constant RPM at low pedal positions proved difficult. A forum post from a TL owner (TL models of similar years use the same APP sensor) described erratic throttle body butterfly valve movement at low APP ranges. This description resonated perfectly with the engine behavior I was experiencing. There were even instances of unstable idle RPM when the car was stationary.
To cut to the chase, I ordered a new APP sensor module (part number 37971-RBB-003) online. The replacement arrived promptly, and the installation process was surprisingly straightforward, taking only about 15 to 30 minutes. While I’m not a seasoned mechanic, and my typical DIY car work is limited to simple tasks like O-ring replacements, this sensor swap was well within my capabilities. The tools required were minimal: a 10mm socket with an extension and a 12mm open-end wrench.
The result after replacing the APP sensor was transformative. Driving with the new sensor felt like a completely different car. Smooth starts were effortlessly restored, and the engine ran noticeably smoother at lower RPMs. The odd transmission behaviors I had previously observed vanished entirely. The improvement was so significant that it highlighted how gradually the problem had worsened, becoming almost normalized over time.
For Acura TSX owners facing P2138 codes and similar driveability issues, replacing the APP sensor is a DIY repair worth considering. While OBD2 scanner apps can be helpful for initial diagnostics, sometimes the solution lies in addressing a specific component. This experience underscores the value of understanding your car’s systems and being willing to tackle simple repairs yourself.
