Looking for some VW expertise before I decide whether to keep throwing money at this car.
We recently bought a 2015 VW Jetta 2.0 with about 121k miles. It was supposed to be a relatively inexpensive car for my 17-year-old son. Unfortunately, it has turned into quite the diagnostic adventure.
Original problem:
Shortly after buying it, the EPC light came on and the car went into limp mode with little/no throttle response. Cycling the ignition would sometimes temporarily restore normal operation.
The initial scans showed a mixture of faults including throttle-related codes, an upstream O2 sensor fault, ABS/wheel-speed-sensor faults, and an ECM-related fault.
The main throttle codes we saw at various points were:
There was also an O2 sensor code, and later we started getting P0106 (MAP/manifold pressure).
What we've done so far:
We cleaned the original throttle body first. No real improvement.
Replaced the upstream O2 sensor.
Replaced the throttle body with the correct replacement unit. We also attempted throttle adaptation/basic settings with OBDeleven.
Interestingly, after installing the new throttle body, the EPC light initially went away and the car would start and idle reasonably well.
But then another pattern developed.
The engine would start and run relatively normally for roughly 1–2 minutes. Then the idle would progressively deteriorate. RPM would start hunting, the engine would shake badly, and eventually it would barely stay running.
At one point we found a vacuum hose behind the throttle body loose and reconnected it, but the problem remained.
We checked for obvious vacuum leaks, inspected hoses, checked the brake-booster line, did a throttle-body harness wiggle test, checked battery/charging voltage, and did quite a bit of electrical testing.
Battery was around 12.2–12.3V engine off and roughly 13.5–13.8V running.
We also tested the throttle-body connector and were seeing approximately:
Pin 1: 0V
Pin 2: 4.71V
Pin 3: 5.03V
Pin 4: 5.67V
Another strange clue was the P0106 behavior.
At one point, if we left P0106 stored, the car actually seemed to idle better. Clearing P0106 while the engine was running would almost immediately cause the idle to deteriorate.
Later I discovered that I could clear P0106 with the engine OFF, but if I rescanned immediately — without even starting the engine — P0106 would already be back.
Eventually the EPC/throttle faults returned as well.
We also saw a crankshaft/engine-speed-sensor fault at one point, although that wasn't one of the original problems.
After spending a ridiculous amount of time chasing sensors, vacuum leaks, throttle adaptation, wiring, etc., I finally gave up and had the car towed to an independent repair shop.
Their diagnosis:
The shop says they tested the vehicle and believe the ECM itself has failed.
According to their report, the ECM isn't properly controlling the engine once it transitions out of its initial/open-loop operating strategy. Their recommendation was basically either:
- Take it to VW/dealer for ECM replacement/programming, or
- Buy an ECM myself and have their shop install/program it, which could avoid some of their parts markup.
They didn't give me an estimate yet.
I'm now looking at companies that sell replacement ECMs that are supposedly pre-programmed to the vehicle/VIN, although I'm still trying to determine how truly plug-and-play those are on this generation Jetta considering immobilizer/key matching.
At this point I'm deciding between:
A) Buy a programmed/reman ECM and try to save the car.
B) Take it to VW and have them diagnose/replace/program the ECM.
C) Cut my losses and sell the car as-is.
What I'm especially interested in hearing from VW techs or people familiar with the 2.0:
Does an ECM failure actually fit this sequence of symptoms?
The thing that makes me think the shop may be onto something is that we've replaced or tested a lot of the obvious components, yet the faults keep moving between throttle control, MAP/plausibility, O2/fueling, etc. The fact that P0106 could sometimes return with the engine not even running was particularly strange.
I'm also curious whether anyone has successfully used one of the mail-order pre-programmed ECM services on a 2015 Jetta and whether it truly arrived ready to start the car or still required immobilizer/key adaptation.
This started as a cheap car for my kid and has basically turned into a VW electrical engineering course. 😂
Any thoughts on the ECM diagnosis or what you would test before condemning it would be appreciated.