P0299: Turbocharger/Supercharger "A" Underboost Condition
Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.
What P0299 means
The engine isn't making as much boost as the ECM is asking for.
What sets it
On turbocharged and supercharged engines the ECM sets a desired boost pressure and reads actual boost from a pressure sensor. If actual stays below desired by a calibrated margin for long enough, it sets P0299. Many vehicles reduce power to protect the engine. The cause is often a leak in the charge-air path rather than the turbo itself.
Symptoms
- Low power, especially at higher load
- Reduced-power or limp mode
- Whistle or hiss from a boost leak
- Possible smoke if the turbo is damaged
Is it safe to drive?
General guidance, not a guarantee: Usually drivable gently in reduced-power mode, but avoid heavy loads. Stop driving if you notice oil smoke, metallic noise or a sudden loss of power, which can mean turbo damage. If in doubt, have it towed and diagnosed.
Common causes
Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.
- Boost leak: split hose, loose clamp, cracked intercooler or charge pipe
- Wastegate stuck open or actuator fault: including vacuum or electronic actuator issues
- Boost pressure sensor or wiring fault
- Restricted intake or exhaust: clogged air filter, restricted catalyst or DPF on diesels
- Turbocharger wear or damage
Test before you replace: diagnostic sequence
General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.
- Compare desired and actual boost in live data during a safe, legal road test, and note when the shortfall starts.
- Check the boost sensor reading key-on, engine-off against barometric pressure. They should agree closely (general guideline; check the manufacturer's data).
- Pressure-test or smoke-test the charge-air system from the turbo outlet to the throttle with suitable adapters, within the manufacturer's recommended pressure.
- Check wastegate or vane actuator operation with the scan tool or the manufacturer's procedure.
- Check for intake and exhaust restrictions.
- Inspect the turbo for shaft play, oil leaks and blade damage only after the system checks above.
- Verify desired and actual boost track after repair.
Bench note: Charge-air leaks hide under heat shields and behind bumpers, and they often only open up under boost. Pressure-test the system before condemning the turbo.
How-to guides for this code
Related codes
FAQ
Does P0299 mean the turbo is bad?
Not usually at first. Boost leaks and wastegate or actuator problems are common. Test the system before replacing the turbo.
Why is my car in limp mode?
Many vehicles cut power when boost is below target, to protect the engine and turbo.
Can a dirty air filter cause P0299?
A badly restricted filter can reduce boost. It is a quick check.
Sources
- SAE J2012, Diagnostic Trouble Code Definitions (standard page; code titles only)
- Generic code title cross-check: obd-codes.com generic code list
Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.