We recently encountered a perplexing issue with a 2016 VW Jetta that arrived at our shop post-auction with a crank-but-no-start condition. The initial diagnostic scan revealed fault codes P0687, U1123 Vw, and P0016. Suspecting a mechanical issue, the customer had already inspected the timing chain and various sensors, all of which appeared to be in good working order. While the engine would briefly run with the introduction of brake fluid into the intake, a sustained start was impossible. Relay checks were also performed, but no faults were found, and relay swapping yielded no improvement.
Initial Symptoms and Troubleshooting
Programming a donor PCM seemed like a logical next step. The programming process itself went smoothly, with immobilizer re-adaptation completing without errors. Aside from tire pressure sensor codes, no new significant faults emerged. However, the no-start issue persisted. Upon pressing the ignition, the Jetta would enter an extended crank cycle, lasting around 30 seconds, but failed to start. At the customer’s request, we re-installed the original PCM and re-adapted it using ODIS. This process also appeared normal, with no additional codes appearing. However, the vehicle then shifted to a no-crank condition altogether.
PCM Replacement and Persistent Cranking Issue
Undeterred, the shop sourced a second donor PCM from a reputable vendor a week later. Programming this unit mirrored the experience with the first donor PCM – successful programming, no new codes related to starting, but the same 30-second crank cycle with no ignition. Frustration mounting, we reverted to the original PCM once more and re-adapted it. This time, the Jetta returned to the no-crank state, and the U1123 VW code was the only fault present.
Understanding the U1123 Code
The U1123 VW code, as we understand it, is a communication fault and is notoriously vague, offering little concrete direction for troubleshooting. It seems to be a symptom rather than a precise diagnosis. We are now seeking insights from the community: Has anyone else encountered a similar scenario with a U1123 VW code on a Jetta or other VW model after PCM replacement, leading to a no-start or no-crank condition? Any guidance or shared experiences would be greatly appreciated as we navigate this diagnostic challenge.
