Issues with boost control can lead to poor performance, engine fault codes, excessive smoke, or even turbo failure. One of the most common faults we see is P0238 – Turbocharger Boost Sensor A Circuit High, which usually indicates excessive boost pressure, not a faulty sensor.
In this guide, we’ll break down common causes of low or high boost — tailored to different turbo systems, including wastegated turbos, VNT systems, position sensor-equipped systems, and fully electronic actuators.
🔧 Common Symptoms and Faults
Low Boost — Potential Causes:
- Faulty N75 solenoid (boost control valve)
- Leaking or split vacuum lines between solenoid and actuator
- Large boost leak in intercooler or pipework
- Blocked DPF restricting exhaust flow
- Jammed VNT vanes (soot build-up)
- Sticking wastegate or actuator
- Incorrect VNT setting on aftermarket turbos
- Weak vacuum supply (bad vacuum pump or reservoir)
- Clutch switch fault – might not actually show a fault, can prevent the turbo from boosting. Common on EDC16C34 systems, especially on Fiesta 1.6 16v
Diagnostic tip:
If the air mass measured is far below the setpoint, and you’re seeing heavy smoke, suspect a large boost leak.
⚙️ Wastegated Turbo – Vacuum-Controlled via N75 – no position sensor
Applies to 1.6 16v HDI 90, old 2.0 HDI 8v 110BHP
🔻 Low Boost:
- Leaking vacuum hoses between the N75 and wastegate actuator
- Faulty N75 solenoid not supplying vacuum
- Sticky wastegate mechanism
- Weak vacuum source (cracked line or bad pump)
- Clutch switch faults – the ECU on an EDC16C34 goes to a default position when clutch is pressured, if the clutch switch is faulty you stay at the default turbo % all the time, thus never making any boost.
Test method:
Temporarily apply direct vacuum to the actuator. If the wastegate moves freely and you regain boost (at part load only!), the issue is upstream — likely a bad solenoid or hose leak.
⚠️ Never drive under full load this way – it will overboost and destroy the turbo.
🔺 High Boost:
- N75 solenoid vent blocked – prevents release of vacuum, keeping the wastegate closed which makes more boost
- Incorrectly set actuator – wastegate opens too late
- P0238 fault – sensor hitting limit due to excessive uncontrolled boost
⚙️ VNT Turbo – Vacuum-Controlled via N75 – no position sensor
Applies to 1.6 16v HDI 110, 2.2 HDI 136 2000 to 2005
🔻 Low Boost:
- Sooted or jammed VNT vanes – stuck open, no boost
- Vacuum leaks or failing N75 solenoid
- Aftermarket VNT incorrectly set (preload wrong)
- DPF blockage choking flow
🔺 High Boost:
- Vanes jammed in closed (high boost) position
- Cheap aftermarket turbo with wrongly calibrated VNT mechanism
- N75 solenoid fault (not enough vacuum or vent port blocked causing full vacuum)
- Rod length misadjusted during rebuild – common mistake
- P0238 logged – not the sensor’s fault, it’s reading accurately
Important note:
You cannot set VNT rod length by eye or measuring — even if you think you’ve matched the old one. The vane arm is welded in a slightly different position on nearly every turbo. Flow bench calibration is essential.
Difficult to diagnose due to lack of turbo position sensor and the abundance of cheap turbos on the market.
⚙️ Wastegated Turbo – With Turbo Position Sensor
Applies to 1.6 8v HDI 92, 1.6 8v HDI 75 / 92, 1.6 BlueHDI 75 / 100
These systems are easier to diagnose with proper scan tools.
✅ Diagnostic Method:
- Compare turbo position setpoint vs. turbo position measured.
- If actual position doesn’t match the set point, it’s likely a vacuum or N75 issue.
- If actual matches setpoint but boost is still wrong, suspect a boost leak or bad turbo, blocked DPF etc.
Possible Faults:
- Vacuum issues, as above
- Failed position sensor (rare but possible)
- Sticking actuator mechanism
Live data is your friend here – it tells the full story.
⚙️ VNT Turbo – With Turbo Position Sensor
Applies to 1.6 8v HDI 115 SID807, 1.6 8v BlueHDI 120, 2.0 HDI 150 163
These systems are easier to diagnose with proper scan tools.
✅ Diagnostic Method:
- Compare turbo position setpoint vs. actual.
- If actual lags or sticks, the system is trying but the vanes may be jammed.
- If actual matches setpoint but boost is still wrong, suspect a sensor fault or incorrect mapping.
Possible Faults:
- Vacuum issues, as above
- Jammed VNT, now easy to confirm via mismatch of measured position
- Failed position sensor (rare but possible)
- Sticking actuator mechanism
Live data is your friend here – it tells the full story.
⚙️ Wastegated or VNT Turbo – With Full Electronic Actuator
Common on newer vehicles (e.g. PSA 1.5 BlueHDI)
These are the most reliable systems overall.
✅ Advantages:
- No vacuum system to fail
- Electronic actuator includes internal feedback loop and position sensor
- Diagnostic software shows target vs. actual vane or wastegate position
Possible Faults:
- Failed electronic actuator motor or gears
- Position sensor fault (not tracking correctly)
- Jammed mechanism inside the turbo
- Internal turbo wear or manufacturing faults on aftermarket units
High-quality live data and actuator tests via dealer or professional tools (e.g. Diagbox, Forscan, FAP app) can usually pinpoint the fault quickly.
🧠 The Key to Diagnosis: Live Data + Logic
Whether you’re chasing low or high boost, a scan tool and some logical troubleshooting will always lead you to the root cause.
Focus on:
- Air mass setpoint vs. measured
- Turbo position setpoint vs. actual (if available)
- Fuel pressure (if it’s low, the ECU will limit boost)
- DPF pressure differential
- Vacuum levels from pump to actuator
And of course, don’t forget to test the N75 solenoid itself. Many behave intermittently when warm and can pass a basic test when cold. Swap with a known good one if in doubt.
💬 Final Word
A proper remap depends on a healthy turbo system. If you’re getting underboost, overboost, or P0238 faults, don’t just throw parts at the problem — diagnose smartly.
