P0045
PowertrainTurbocharger/Supercharger Boost Control A Circuit/Open
The boost control solenoid is the little electrical valve that tells your turbo how much boost to make, usually by controlling the vacuum or pressure feeding the wastegate or the variable vanes. When the ECU spots an open circuit or electrical fault in that solenoid or its wiring, it can no longer trust the boost control, so it sets P0045 and often pulls the turbo back to a safe baseline. The result is a flat, gutless engine that feels like the turbo has gone to sleep, and on a lot of cars it'll drop straight into limp mode.
ⓘ Information only. This page provides general educational information about fault code P0045. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0045 mean?
P0045 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Turbocharger/Supercharger Boost Control A Circuit/Open.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, sometimes with a flashing or limp mode warning alongside it
- • Big drop in power, especially the mid-range shove you'd normally feel pulling onto a motorway
- • Lazy throttle response, the car feels like it's holding its breath when you put your foot down
- • Limp mode kicking in, capping the revs and refusing to make proper boost
- • Worse fuel economy because the engine is working harder for less
- • On some turbos a slightly different whine or whistle, though many are silent with this fault
Possible causes
Causes commonly associated with P0045, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed boost control solenoid itself, the most common cause. The internal winding goes open circuit or the valve sticks, and it's a known wear item on hard-worked diesels
- 2. Damaged or corroded wiring to the solenoid, common on engines where the loom runs near the turbo and cooks over the years
- 3. Loose or corroded connector at the solenoid, or a poor earth in the control circuit
- 4. Split or perished vacuum hoses on vacuum-operated systems, which can confuse the whole boost control picture
- 5. Faulty boost pressure sensor or turbo position sensor feeding bad data into the control loop
- 6. A worn or damaged turbo, sticking vanes on a VNT unit or a seized wastegate actuator
- 7. A faulty ECM or PCM, but this is rare and only after everything else checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Get the solenoid's resistance with a multimeter across its terminals. An open winding reads infinite, a good one is usually a few ohms to low tens of ohms depending on the unit, so check against the spec for your engine
- 2. Inspect the wiring and connector right at the solenoid for green corrosion, chafing, melted insulation or pushed-back pins. Wiggle-test it while watching live data if you can
- 3. Check the 12V supply at the solenoid plug with the ignition on. No feed points to a wiring or fuse issue rather than the solenoid
- 4. Go over the vacuum hoses and the wastegate or vane actuator for splits and disconnections if your engine runs a vacuum setup, since a leak here mimics the symptoms
- 5. Scan for other stored codes. P0045 often turns up next to P0234 over-boost or P0299 under-boost, and those tell you whether the turbo is actually fighting the control
- 6. If electrics and vacuum all pass, inspect the turbo itself for sticking vanes, a seized actuator rod or play in the shaft before condemning anything expensive
Common questions about P0045
Can I sort this myself in the driveway, or is it a garage job? +
You can do a fair bit yourself. The solenoid is usually a bolt-on part costing roughly £30 to £90 for the average UK diesel, and it's often mounted somewhere accessible near the turbo or inlet. If your resistance test shows the solenoid is open or way off spec, swapping it is a straightforward job with hand tools. Where it gets fiddly is a wiring fault. Chasing a broken or corroded wire in the loom takes a multimeter, a wiring diagram and a methodical head, and that's where most people end up handing it to a garage.
If I just clear the code, will it stay gone? +
Only if the fault was a one-off glitch, which it rarely is on this code. If the solenoid winding has gone open or a wire's broken, the ECU will re-flag it within a drive cycle or two, usually as soon as it asks the turbo for boost and gets nothing back. Clearing it is fine as a test to confirm you've actually fixed something, but it's not a cure. A P0045 that keeps returning is telling you there's a real electrical fault to find.
What's the harm in driving it like this for a while? +
The car will limp along, just slowly and without much go. The bigger worry is what's happening to the turbo. With the boost control disabled the engine may over-boost or under-boost, and sustained over-boost can lead to detonation, overheating and damage to the turbo or engine internals over time. A short drive home is fine. Months of ignoring it while the turbo runs uncontrolled is asking for a far bigger bill, so get it diagnosed sooner rather than later.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →