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2018 WRX Misfire and Underboost Issue - P0301 and P0299

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Mike T. · rep 0 · May 25, 2:14 AM · 1 replies · solved · Question

Vehicle: 2018 Subaru WRX Base

Hey guys, my 2018 Subaru WRX (stock, 55,000 miles) has been acting up. The check engine light came on, and I'm getting codes P0301 (Cylinder 1 Misfire Detected) and P0299 (Turbocharger/Supercharger Underboost).

I've noticed a significant loss of power, especially under acceleration, and the engine feels rough at idle. It almost feels like the car is struggling to build boost. I checked the spark plugs on cylinder 1 and they look okay, but I'm not sure what else to check. Could these two codes be related? Any ideas on where to start looking?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:14 AM

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ClearTheCode admin answer:

Experiencing P0301 (Cylinder 1 Misfire) and P0299 (Turbocharger/Supercharger Underboost) simultaneously on your 2018 WRX is concerning and indicates a significant issue within your engine's performance and boost system. These codes are very likely related.

Possible Causes for P0301 (Misfire):

  1. Ignition System: Faulty spark plug (even if it looks okay, it could be failing), bad ignition coil for cylinder 1, or wiring issues.
  2. Fuel System: Clogged or failing fuel injector for cylinder 1, low fuel pressure, or contaminated fuel.
  3. Compression Issues: Less likely, but low compression in cylinder 1 due to valve problems or piston ring wear can cause misfires.

Possible Causes for P0299 (Underboost):

  1. Boost Leak: The most common cause. A crack in an intercooler hose, a loose clamp, a damaged bypass valve (BPV) or blow-off valve (BOV), or a leak in the intake manifold can all lead to boost loss.
  2. Wastegate Actuator Issues: A faulty wastegate actuator or a disconnected wastegate arm can prevent the turbo from building proper boost.
  3. Turbocharger Failure: A failing turbocharger (e.g., worn bearings, damaged compressor/turbine wheels) could be the root cause, but this is usually accompanied by distinct noises or smoke.
  4. MAP/MAF Sensor: A dirty or failing Manifold Absolute Pressure (MAP) sensor or Mass Air Flow (MAF) sensor can cause incorrect air measurements, leading to improper boost control.

How Misfire and Underboost Can Be Related:

A significant boost leak (P0299) can cause the engine to run lean, which can, in turn, lead to misfires (P0301) as the air-fuel mixture becomes too lean to ignite properly. Conversely, a severe misfire might cause the engine management system to pull timing or reduce boost as a protective measure.

When to See a Shop:

  • Immediately: Driving with a persistent misfire and underboost can cause further engine damage, particularly to the catalytic converter. Do not delay diagnosis.

What a Shop Will Do:

  1. Comprehensive Diagnostic Scan: Beyond reading codes, they'll check live data for fuel trims, boost pressure, MAF readings, and misfire counts.
  2. Boost Leak Test: Pressurize the intake system to pinpoint any leaks.
  3. Ignition System Check: Test spark plug and coil pack for cylinder 1, potentially swapping components to other cylinders to isolate the fault.
  4. Fuel System Check: Verify fuel injector operation and fuel pressure.
  5. Wastegate Inspection: Check the wastegate actuator and arm for proper function and connection.
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