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2020 Civic Si Losing Boost, P0299 Code Popping Up

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Mike T. · rep 0 · May 25, 1:46 AM · 1 replies · solved · Question

Vehicle: 2020 Honda Civic Si

Hey guys,

My 2020 Honda Civic Si, with about 40,000 miles, has been acting up. Lately, I've noticed a significant loss of power, especially when I try to accelerate hard. It feels like the turbo isn't kicking in as it should. The car just feels sluggish and unresponsive.

To make matters worse, the check engine light came on, and when I scanned it, I got a P0299 code, which points to 'Turbocharger/Supercharger Underboost'. I've tried clearing it, but it comes back after a bit of driving.

Could this be a wastegate issue, a boost leak, or something more serious with the turbo itself? I'm hoping it's not a super expensive fix. Any common culprits for this code on the 1.5T engine in the Si?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 1:46 AM

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ClearTheCode admin answer:

The P0299 code ('Turbocharger/Supercharger Underboost') on your 2020 Civic Si indicates that the engine's control module is detecting less boost pressure than expected. This is a common issue with turbocharged engines and can have several causes:

  1. Boost Leaks: This is often the most common culprit. Check all intercooler piping, charge pipes, and vacuum lines for cracks, loose connections, or damage. Even a small leak can significantly reduce boost pressure.
  2. Wastegate Actuator/Valve: The wastegate controls exhaust flow to the turbo. If the wastegate actuator (or its linkage) is faulty, stuck open, or not operating correctly, it can prevent the turbo from building full boost. On some Honda 1.5T engines, issues with the wastegate actuator or its electronic control have been noted.
  3. Turbocharger Failure: While less common, the turbocharger itself could be failing. This could involve worn bearings, damaged turbine/compressor wheels, or issues with the variable geometry components (if applicable).
  4. MAP/Boost Sensor: A faulty Manifold Absolute Pressure (MAP) sensor or boost sensor could be providing incorrect readings to the ECU, leading it to believe there's an underboost condition when there isn't, or vice-versa.
  5. Exhaust Leaks: Leaks in the exhaust manifold or upstream of the turbo can reduce the exhaust energy available to spin the turbo, leading to underboost.

When to see a shop:

  • Immediately: If you notice excessive smoke from the exhaust, strange noises from the turbo (whining, grinding), or a significant drop in engine performance. Continuing to drive with a severe underboost can potentially cause further engine damage.
  • Soon: If you're unable to locate a boost leak, or if visual inspection doesn't reveal any obvious issues. A qualified mechanic, especially one familiar with Honda's 1.5T engine, can perform a smoke test to find leaks, check wastegate operation with a diagnostic scanner, and accurately diagnose the source of the P0299 code. Specialized tools are often needed to properly diagnose turbo system issues.
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