One of the most common turbocharger fault codes on Mercedes diesels. It means the ECU is detecting less boost pressure than demanded — and it can cause significant power loss and limp mode.
P0299 means the ECU is seeing boost pressure that is significantly below the target value — typically 1.0–1.5 bar absolute on most Mercedes turbocharged engines. On most cases, this is a boost leak or a control issue, not a failed turbocharger. It should be diagnosed cheapest-first.
Do not let anyone sell you a new turbo before the boost system and control components have been inspected. Full diagnostic order below.
In a turbocharged Mercedes engine, the ECU uses the boost pressure sensor (often integrated into the charge air cooler or intake manifold) to monitor the pressure produced by the turbocharger. The ECU compares the actual boost to a target value based on engine load and RPM. P0299 is logged when the actual boost falls below the target by a set margin for a set period. Crucially, the code describes an underboost condition — the turbo is not producing enough pressure. This could be because of a boost leak (split pipe, loose hose, faulty intercooler), a stuck wastegate or variable geometry actuator, a faulty boost pressure sensor, or a turbocharger mechanical issue. Underboost is frustrating because it kills power, but it rarely damages the engine itself. It appears across the full range of Mercedes turbocharged engines, from the OM651 diesel to the M276 petrol V6, and the diagnostic approach is essentially identical regardless of which engine triggers it.
Ordered by how often each turns out to be the real culprit — with typical fitted cost against each, so you can sanity-check any quote you've been given.
| Cause | Likelihood | Typical Cost (fitted) | Notes |
|---|---|---|---|
| Boost leak (split hose, loose clamp, cracked intercooler) | Very High | £150–£400 | Most common — a simple visual inspection can find it |
| Faulty boost pressure sensor (MAP sensor) | High | £100–£250 | Reports falsely low pressure; turbo may be fine |
| Stuck / sticking wastegate or VNT actuator | Medium | £200–£600 | Carbon buildup or mechanical seizure — common on higher-mileage units |
| Boost control solenoid / vacuum leak | Medium | £150–£350 | Faulty solenoid or leaking vacuum lines that control the actuator |
| Turbocharger mechanical failure (worn bearings, damaged vanes) | Low | £1,200–£1,800 | Genuine turbo failure is rare — rule out everything above first |
Cost figures [PATTERN DATA] — indicative independent-specialist pricing, not drawn from our engine replacement dataset.
With key on, engine off, compare the boost sensor reading to atmospheric pressure. If it's way off, the sensor is the most likely culprit.
Look for splits, cracks, or loose clamps. A simple visual inspection can find the leak.
Check the vacuum hoses to the wastegate/VNT actuator and the electrical connections to the boost solenoid. A simple leak or bad connection can cause underboost.
Using a vacuum pump, apply vacuum to the actuator. It should move freely. If it's stuck, it needs cleaning or replacement.
If all the above are working correctly and boost is still low, the turbo's mechanical condition may be the issue.
Our diagnostic wizard walks the same order, or confirm your engine first with the registration lookup.
Conditionally, yes — short journeys, but avoid heavy acceleration. P0299 on its own is not an engine-destroying fault, but it will result in reduced power and fuel economy. If the vehicle has dropped into limp mode, motorway driving is unsafe because you cannot accelerate normally. Get it looked at within a day or two, and drive gently with minimal boost demand.
Yes, for short journeys, but avoid heavy acceleration. The car will have reduced power and may go into limp mode. Get it checked within a few days.
No. In fact, turbo failure is the least likely cause. A boost leak or a faulty sensor are far more common and much cheaper to fix.
Only temporarily. The code will return as soon as the ECU detects underboost again. Clearing it also destroys the freeze-frame data a specialist would use to narrow the cause.
Expect one hour of diagnostic time at an independent Mercedes specialist. Many will offset that cost against the repair if you proceed with them.
Cheapest-First AlwaysWe rank causes by likelihood and cost, not by what's profitable to quote.
Honest About CostWhere a fault is usually cheap to fix, we say so plainly.
15+ Vetted Mercedes SpecialistsDiagnostics carried out by people who work on these daily.
100% IndependentNo sponsors, no manufacturer influence.
Compare diagnostic and repair quotes from our vetted specialist network.
COMPARE QUOTES NOW → RUN THE DIAGNOSTIC WIZARD