Case Study: 2017 Subaru Forester - Misfire and Second Opinion on Headgasket

A local driver recently visited our downtown Louisville repair facility at 422 E. Broadway to take advantage of our free second opinion offer. They were driving a 2017 Subaru Forester equipped with the 2.5-liter Boxer engine. Another local repair shop had evaluated a rough running condition and immediately recommended a complete cylinder head gasket replacement. Subaru Boxer engines have a historical reputation for head gasket issues in older generations, leading many technicians to jump to expensive conclusions based on vehicle badge alone. Our shop philosophy requires us to verify failures objectively before condemning major engine components.

The Symptoms

The vehicle owner reported the following concerns during their initial consultation:

• An illuminated check engine light present on the dashboard.

• A noticeable engine misfire causing a rough idle and hesitation during acceleration.

• A previous shop's diagnosis claiming catastrophic head gasket failure.

Our Diagnostic Process

At Auto Lab, our "test, don't guess" approach is critical for saving our clients from unnecessary financial burden. To evaluate the previous shop's head gasket diagnosis, we performed a stringent series of tests. First, we conducted a combustion leak test (also known as a block test) to detect the presence of exhaust gases in the cooling system. The test fluid remained perfectly blue, indicating absolutely no breach in the cylinder head gaskets. Next, we attached a cooling system pressure tester with the engine at operating temperature to check for external or internal leaks. The system held pressure flawlessly, confirming the engine's structural integrity.

The Root Cause and The Fix

With the cylinder head gaskets definitively cleared of any faults, our team focused on the actual source of the misfire. We connected our professional diagnostic scan tool and retrieved the diagnostic trouble codes, which pointed directly to a specific cylinder misfire. By monitoring live misfire data and swapping the ignition components between cylinders, we isolated the failure to a single bad ignition coil. The ignition coil is responsible for multiplying battery voltage to create the spark that ignites the fuel. When an ignition coil fails, the cylinder stops firing entirely, resulting in the rough engine feel and the check engine light.

We replaced the defective ignition coil and installed a fresh set of spark plugs to restore optimal combustion. However, we wanted to provide absolute peace of mind regarding the head gaskets. To completely rule out coolant mixing with the engine lubrication system, we performed a full synthetic oil change and sent a sample of the used engine oil to an independent laboratory for fluid analysis. This testing physically ruled out any microscopic coolant contamination in the oil. The engine ran perfectly smooth on our post-repair test drive, and the customer avoided a massive, unnecessary repair bill.

The SBC Auto Lab Takeaway

If a repair shop tells you your vehicle needs a major engine repair based purely on its brand reputation, always secure a second opinion. A misfire and a check engine light can stem from dozens of different components, and proper diagnostic testing is the only way to find the truth. We invite Louisville drivers to bring their vehicles to our facility on E. Broadway for an honest, tool-verified evaluation before authorizing thousands of dollars in major engine work.