The brief
Norman's A4 had the check engine light on. A check engine light has a long list of possible causes, and the only way to find the right one is a proper diagnostic scan rather than a guess. The intake manifold has a position sensor that reports what the manifold's runner flaps are doing, so the engine management knows the air path and can fuel accordingly. When that sensor fails or drifts, the reading goes off, the management can't trust it, and it sets a fault code and lights the dash. A failed sensor doesn't recover, so it gets replaced, and on this engine that means the intake manifold has to come off first, which is a tedious job, but it's the proper way to fit the new sensor.
The diagnosis
The precision diagnostic scan pointed to a fault in the intake manifold position sensor, which is why the light was on. The rest of the intake and the engine were sound, the sensor was the fault. That's an intake manifold position sensor replacement, which means the manifold off first, with the adaptations reset, rather than chasing a check engine light without knowing the cause.
The work
The intake manifold was removed, the tedious part, the faulty intake manifold position sensor replaced with a new genuine Audi-spec sensor, and the manifold refitted with fresh gaskets, every fastener torqued to the manual figures. The fault code was cleared, the engine's adaptations reset so it relearns on a sensor it can trust, and the air filter checked. A road test confirmed the light stayed off, a steady idle, smooth running through the rev range, and the fuel trims in range.
The outcome
No check engine light, a steady idle, smooth running, full power, and the fuel trims back in range. Norman got the A4 back running properly with the light out. Scanning it rather than guessing meant we replaced the sensor that was actually faulty, so it was one visit and done, engine fault code history.
