BMW fault code 30FE / P0234 (N54, N55): turbocharger overboost — when pressure arrives and nobody asked

30FE is overboost — the mirror of 30FF. Same monitor, opposite failure: the DME tried to vent drive from the turbo and pressure kept climbing past the specified envelope. SAE name P0234. It's rarer than 30FF on a stock car and much more common on tuned ones.

What the factory document says

The trigger window is identical to underboost's — 1,900–7,000 rpm, specified-to-barometric ratio between 1.2 and ~1.5 as a function of engine speed, 2.5 s after injector equalization, barometric-dependent timing term — but tripped by pressure above envelope rather than below. MIL off, EML on (ID 29). The symmetry matters: both codes are the same control-loop sanity check, read from opposite ends.

What usually actually causes it

Community ranking, paraphrased:

1. 01**A wastegate that can't open** — seized or galled linkage, or an actuator that's lost its vacuum supply, means the ECU has no vent and boost runs away. This is the mechanical classic.
2. 02**Solenoid or line faults on the control side** — a stuck wastegate solenoid, pinched line, or cracked vacuum hose that happens to fail in the "gate closed" direction.
3. 03**Tune/boost-solenoid wiring mistakes** — on aftermarket setups (MAC solenoid conversions, budget boost controllers), reversed ports or wrong line routing hold the gate shut. [Duty at 0% with pressure climbing](../what-wgdc-means) is the fingerprint.
4. 04**Boost sensor or its reference line** lying — rare, but a sensor reading low makes the DME command more until the real pressure overshoots everywhere else.

How to read it in a log

One pull, boost actual vs target plus WGDC↗: actual overshooting target while WGDC drops toward zero = the DME is trying to vent and can't — gate, solenoid, or lines. WGDC not moving at all = control-side fault.

The fix path

• Inspect the wastegate linkage for freedom and the actuator lines for cracks — the cheap stuff first.
• Aftermarket solenoid? Verify port assignment and line routing against the [MAC solenoid install guide](../mac-solenoid-install-guide) before blaming turbos.
• Recurring overboost on stock hardware with clean lines is a hard look at the actuator/diaphragm.
• The underboost twin ([30FF](../fault-code-30ff)) covers the leak/rattle side of the same system.