P0045 Code: Turbo/Supercharger Boost Control Solenoid A Circuit/Open
The engine computer sees an open or shorted circuit to the solenoid that controls turbo boost.
At a glance
Severity: Moderate
- Meaning
- Turbo/Supercharger Boost Control Solenoid A Circuit/Open
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $5–$1,800
- DIY difficulty
- Moderate
On most turbocharged engines the engine computer doesn't move the wastegate or variable-vane actuator directly. It pulses a small electric solenoid (the N75 valve on VW and Audi) that meters vacuum or boost pressure to the actuator, and it monitors the current in that circuit every time it switches the solenoid. P0045 is a purely electrical code: the computer switched the driver on and saw no load, or saw a short, so it knows the solenoid coil or its wiring is faulty. This is different from P0046, where the circuit is intact but boost doesn't respond. Without a working solenoid the actuator falls back to its spring position, so a wastegate engine only makes base spring boost and a vacuum-operated variable-vane diesel makes almost none, which is why the car usually feels flat and goes into limp mode.
Can I keep driving? The engine will run, but with low boost and often in limp mode, so keep speeds modest and avoid passing and towing. Because the fallback position is the safe one, there's no overboost risk, but the wiring fault should be found before it causes bigger problems.
How to read P0045
- P Powertrain
- 0 Generic (SAE)
- 0 Fuel/air & emissions (aux.)
- 45 Specific fault
Symptoms
- Check engine light on, often right after start-up
- Weak acceleration, turbo never seems to spool
- Limp mode with capped rpm or throttle
- No clicking or buzzing from the boost solenoid when the engine is running
- Diesel engines: black smoke under load because fuel is delivered without boost
- Sometimes intermittent: full power returns after a restart, then drops again
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Boost control solenoid coil failed open or shorted internally (heat-aged from its spot near the turbo)
- Chafed, melted or broken wiring between the ECM and the solenoid, typically where the loom passes the exhaust side
- Corroded, loose or oil-soaked connector at the solenoid
- Blown fuse or failed relay on the solenoid's supply side (often shared with other engine solenoids)
- Aftermarket solenoid with the wrong coil resistance, or a manual boost controller that removed the solenoid from the circuit
- Water intrusion after an engine wash, or rodent damage to the harness
- Faulty ECM driver stage (rare; only after the circuit is proven good)
Diagnosis step by step
Confirm the failure is electrical
Read all codes and the freeze frame. P0045 on its own with boost stuck at a fixed low level points to the circuit; if P0046 or P0299 accompany it, the computer also saw the boost response fail. On a scan tool, desired boost might read 15 to 20 psi under load while actual boost sits around 5 to 7 psi on a wastegate engine, or barely above atmospheric on a vacuum-operated variable-vane diesel.
Tools: OBD-II scanner
Command the solenoid and listen
Find the solenoid (a small plastic valve with two or three hose ports near the turbo or on the firewall) and run the scan tool's output or actuator test. A healthy solenoid ticks or buzzes audibly. Silence with the test active means no current is flowing.
Tools: OBD-II scanner
Check the supply side
Unplug the solenoid and, with the key on, measure between the supply pin and battery negative. You should see battery voltage, around 12.4 V or higher. No voltage means a fuse, relay or supply wire fault, and the fuse box diagram will show which fuse feeds it.
Tools: Multimeter
Measure the coil resistance
Measure resistance across the two solenoid pins. Most boost control solenoids fall between roughly 20 and 40 ohms, though some sit as low as 10; check the spec for your engine. An open reading means the coil has failed, and a reading near zero means it's shorted. Either way the solenoid is the fault.
Tools: Multimeter
Test the control wire from the ECM
With the solenoid unplugged, connect a test light or LED between the supply pin and the control pin of the harness connector and run the actuator test again. The light should pulse. Never lit means an open control wire or a bad ECM driver; always lit means the control wire is shorted to ground.
Tools: Multimeter
Inspect the harness route
Trace the wiring from the solenoid back toward the main loom, especially where it runs past the turbo heat shield, exhaust manifold and any sharp bracket edges. Wiggle the loom while watching the live data or test light for dropouts. Look inside the connector for green corrosion, oil from blow-by or a backed-out pin.
Tools: Flashlight, Multimeter
Verify the repair under load
After the fix, clear the code and drive with the scan tool graphing desired versus actual boost. On a working system actual boost should follow the target within about 1 to 2 psi through the mid-range, and the solenoid duty cycle should move with load instead of sitting at 0%.
Tools: OBD-II scanner
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace blown fuse | Yes | $5–$15 |
| Repair chafed wiring or corroded connector | No | $60–$250 |
| Replace boost control solenoid (N75-type valve) | Yes | $90–$350 |
| Replace damaged engine harness section | No | $250–$700 |
| Repair or replace ECM (rare) | No | $600–$1,800 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Volkswagen
On VW and Audi 1.8T, 2.0T and TDI engines the boost control solenoid is the N75 valve. The valve itself and its connector live in a hot area, and cracked or brittle wiring at the connector is a common finding, so check the plug and the last few inches of wire before ordering a new valve.
Emissions / smog test impact
Will fail
A lit check engine light is an automatic failure in states with OBD-II emissions testing. Fix the circuit and drive enough to complete the readiness monitors before retesting.
Frequently asked questions
What does the boost control solenoid actually do?
It's a small electrically pulsed valve that regulates how much vacuum or boost pressure reaches the wastegate or variable-vane actuator. The engine computer changes the pulse width to fine-tune boost. If the solenoid has no power, the actuator simply sits at its spring position.
Is P0045 the same as P0243?
They overlap but aren't identical. P0243 refers specifically to a wastegate solenoid, while P0045 covers the general boost control solenoid, including the one that drives variable-vane turbos on diesels. Which code your car sets depends on how the manufacturer labeled the circuit.
Can I just unplug the solenoid and drive on spring pressure?
You can, and many cars will run that way, but the check engine light stays on, the computer keeps limiting fuel and torque and a diesel with a vacuum-controlled variable-vane turbo may produce hardly any boost at all. It's a get-home measure, not a fix.
Why did the code appear right after an engine bay wash?
Water in the solenoid connector can bridge the pins or corrode them, and a wet coil can short. Let the connector dry, clean it with contact cleaner and add a little dielectric grease. If the code returns dry, the solenoid probably took damage.
Will an aftermarket manual boost controller cause P0045?
Yes, if it was installed by removing or unplugging the factory solenoid. The engine computer expects to see the coil in its circuit. Some tuners solve this by leaving the solenoid plugged in electrically but disconnected pneumatically, which is not something we recommend on a daily-driven car.