The largest online repair guide library for every car

About us
Contact support
FAQ
Sign up
Sign in

2018 C300 Losing Power, P0299 Code Under Boost

0

Mike T. · rep 0 · May 25, 3:05 AM · 1 replies · solved · Question

Vehicle: 2018 Mercedes-Benz C300

Hey everyone,

I've been experiencing a strange issue with my 2018 Mercedes-Benz C300. Lately, I've noticed a significant loss of power, especially when accelerating onto the highway or trying to pass. It feels like the turbo isn't kicking in properly.

The Check Engine Light came on a few days ago, and when I scanned it, I got a P0299 code, which indicates 'Turbocharger/Supercharger Underboost'. Has anyone else dealt with this on their C300 or similar Mercedes model? What was the fix?

It's a bit frustrating as the car only has about 60,000 miles on it. Any advice on where to start looking would be greatly appreciated before I take it to the dealership.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:05 AM

0

ClearTheCode admin answer:

The P0299 code ('Turbocharger/Supercharger Underboost') on your 2018 Mercedes-Benz C300 indicates that the engine's boost pressure is lower than the specified target. This often leads to reduced power and can be caused by several factors within the engine and drivetrain system.

Here are the common causes and steps to consider:

  1. Boost Leak: This is one of the most frequent culprits. Check all turbocharger hoses, intercooler piping, and connections for cracks, loose clamps, or disconnections. Even a small leak can significantly affect boost pressure.
  2. Wastegate Actuator/Valve Issues: The wastegate controls exhaust flow to the turbocharger. If it's stuck open, faulty, or its actuator (electronic or vacuum-operated) isn't functioning correctly, the turbo won't build sufficient boost.
  3. Turbocharger Failure: While less common at 60,000 miles, the turbocharger itself could be failing. Look for excessive shaft play, oil leaks, or unusual noises.
  4. Diverter Valve/Bypass Valve: This valve releases excess boost pressure. If it's faulty and stuck open, boost will escape, leading to underboost.
  5. N75 Valve (Boost Pressure Control Valve): This solenoid controls vacuum to the wastegate actuator. A malfunctioning N75 valve can prevent proper wastegate operation.
  6. Clogged Catalytic Converter or Exhaust Restriction: Although less direct, a restricted exhaust can hinder turbocharger performance by creating back pressure.

When to see a shop:

It's highly recommended to have your vehicle inspected by a qualified Mercedes-Benz specialist or an independent shop familiar with these engines. Diagnosing boost issues often requires specialized tools to perform smoke tests for leaks, monitor live data (boost pressure, wastegate duty cycle), and check component functionality. Driving with persistent underboost can lead to further engine strain or performance issues. A professional can accurately pinpoint the root cause and ensure proper repair.

Menu

Help & account