P0299 on Mercedes-Benz: How to Diagnose & Fix

The plastic turbo resonator (charge air pipe) on OM642-equipped Sprinters cracks from heat cycling and vibration, causing a significant boost leak. The engine loses power dramatically, especially under load or when climbing grades. This is one of the most common failure items on pre-2019 Sprinters and typically occurs between 60,000-100,000 miles.

Sprinter diesels use a variable-geometry turbo whose vane position is controlled by an electronic actuator (PWM-driven on the OM642; vacuum/wastegate-controlled on some OM651). The actuator's electronics or worm-drive wear out, so the ECU can no longer command correct boost. The van loses power intermittently and falls into limp mode, often with under/over-boost codes. A complication on the OM642: each actuator carries vehicle-specific programming, so an un-coded replacement will itself throw a fault and stay in limp mode.

The M274 2.0L turbo engine develops an audible wastegate rattle at idle and low RPM due to wear in the wastegate actuator linkage. While not immediately dangerous, it worsens over time and can eventually affect boost control.

The W166 GLE 250d and GLE 350d (2015-2019, OM651 2.1L twin-turbo / OM642-related diesels) have a documented pattern of turbocharger bearing collapse and EGR cooler failure, typically appearing between roughly 75,000 and 95,000 miles. Frequent short trips and DPF regeneration cycles cause oil coking in the turbo, leading to sudden power loss, limp mode, whining/grinding and blue smoke. Separately, thermal stress cracks the EGR cooler, contaminating engine oil with coolant (milky oil-cap residue, unexplained coolant loss, misfires). Delphi piezo injector coking on the OM651 compounds rough running and misfires. This is distinct from the routine DPF-regeneration issue already catalogued.

On the X253 GLC 300 the vacuum-operated turbo wastegate actuator uses a rubber diaphragm that hardens and tears after repeated heat cycling. Once it splits, the actuator can no longer hold vacuum, the wastegate hangs open, and the engine never builds commanded boost. The ECU logs an underboost fault and drops the car into reduced-power limp mode, often intermittently at first (a flat spot on hard acceleration, sometimes a rattle or hiss from the turbo area) before becoming a hard fault. This is specific to the 2.0-liter four-cylinder turbo engines (M274 in earlier cars, M264 in the 48V-equipped later ones) and is NOT the same failure as the camshaft adjuster rattle or the PCV/crankcase-vent problem, both of which sit on the same engine but have different symptoms and different repairs. Owners are frequently quoted for a whole turbocharger when only the actuator has failed.

The M133 turbo's wastegate is actuated by a mechanical rod and flapper valve that runs at very high exhaust gas temperatures. On GLA45 (and its CLA45/A45 siblings) the flapper bushing and actuator rod wear until there is visible free play, so the wastegate no longer seals the bypass ports. Boost bleeds past, the ECU sees target boost it cannot reach, and it stores P0299 (turbocharger 'A' underboost) and cuts the car to limp/reduced-power mode - often intermittently, restoring full power after a restart until it happens again. Owners describe rattling from the turbo at part throttle and a sudden dead-pedal on a pull. Dealers routinely quote a complete turbo assembly ($5,500-6,000), but the failure is usually confined to the wastegate hardware.

On 2014-2015 CLA45 AMG (C117, M133 2.0L turbo) Mercedes-Benz USA opened Service Campaign 2014120001 in January 2015 after turbochargers failed prematurely because contamination in the turbo oil feed line starved the shaft bearings. Dealers measure turbo shaft axial play; more than 0.08 mm indicates bearing wear and the turbo plus oil feed lines are replaced. The estimated replacement rate was 25 percent. Owner threads on mercedescla.org and MBWorld describe two waves of the campaign, sudden power loss, whining and smoke, and dealers replacing turbos under warranty. Not a safety recall, so it was done at the next dealer visit rather than by mandatory notice; this is distinct from the M270 turbo oil-seal leak already documented on the CLA250.