P0299 Turbocharger Underboost on Smart Fortwo 451
P0299 on a 451 Brabus or CDI is an underboost code. The usual cause is a boost leak — a split intercooler hose or a loose clamp on the charge piping. Pressure-test the charge side before suspecting the turbo itself.
Typical Symptoms
- Check engine light with code P0299
- Noticeable lack of power, especially uphill or merging
- Hissing, whistle, or chuff under throttle
- Limp mode after a hard pull
- Boost gauge or scan tool reads below target
What it means
P0299 means the ECU asked the turbo for a boost number and never got there. Either the air is escaping before it reaches the cylinders, or the wastegate is stuck open and the turbo cannot build pressure in the first place.
On a 451 the answer is almost always a leak in the charge piping. The turbo itself is rarely the problem on a P0299.
Likely causes, cheapest first
- Split or cracked intercooler hose. Heat cycles age the rubber boots between the turbo, intercooler, and throttle body. A pinhole or split in any one of them lets boost escape. Often visible just by lifting the hose to inspect the underside.
- Loose hose clamp on the charge piping. A clamp that has backed off lets the hose creep off the pipe under boost. Tighten or replace with a T-bolt clamp.
- Wastegate stuck open. Less common than the overboost direction, but a wastegate that will not close starves the turbo of exhaust energy. The actuator rod gives this away if you check it by hand.
- Turbo internals. Last on the list. Worn turbo bearings or damaged compressor wheel show up as oil burning, shaft play, and a bigger noise change, not just P0299 on its own.
DIY check steps
- Visually inspect every charge hose. With the engine off, run your hand along the underside of each rubber boot between the turbo, intercooler, and throttle body. Any split, oil ring, or soft spot is a leak. Replace the hose, don't try to patch it.
- Pressure-test the charge pipe. A simple boost-leak tester is a capped fitting that takes a shop-air feed. Pressurize the intake to about 15 psi with the engine off and listen. Audible hiss locates the leak in seconds.
- Check clamps. Walk every clamp on the charge side and snug it. Old worm-drive clamps lose tension over time; T-bolt clamps hold better.
- Move the wastegate actuator rod. Same check as P0234. If the rod sits flopped open at rest or barely returns, the actuator is your fault, not the turbo.
When to call a shop
If the charge piping holds pressure, every hose is sound, the wastegate moves correctly, and P0299 still returns under load, get a shop to data-log requested vs. actual boost and put a scope on the boost sensor. A real turbo problem is the last suspect, not the first.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Intercooler hose (charge pipe) | $30-90 | Search Google |
| T-bolt or worm-drive clamp set | $8-25 | Search Google |
| Wastegate actuator | $140-320 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread; call a Smart-experienced shop for an actual quote.
Manual references
- Service manual on Manualslib — external mirror (we don't host this specific document).
- Browse Smart manuals on smartcarmanuals.com — pick your chassis code section on the home page if a specific manual isn't listed above.
Community references
- Evilution: Specific error codes (Smart-known patterns)
- Evilution: OBD-II error code reference
- SmartCarOfAmerica: P0299 threads
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