2007 Subaru Outback H6 3.0L - P0174 Code: Is It a MAF or Intake Manifold Vacuum Leak?

Since you purchased the vehicle from a GMC dealership, it's worth considering taking it back for evaluation. However, be aware that the car was sold 'as-is' without warranty — legally they aren't obligated to fix issues immediately. That said, morally, they should address problems arising within days of purchase.

The dealer has diagnosed a faulty O2 sensor, but based on technical research, this is unlikely to be the root cause. They’ve ordered the part and haven’t yet completed any work or confirmed repair costs. I’m trying to gather information so I can ask informed questions during discussions with them. I suspect a vacuum leak — possibly from an intake manifold O-ring — as the most probable issue. Several online sources confirm that such leaks commonly trigger P0174 in non-turbo Subaru models. The salesperson was dismissive, stating it's 'what extended warranties are for' and that the car is fine until driven. I remain hopeful they’ll resolve this fairly, but I want to be prepared for potential overcharging or misleading repairs.

I recommend taking your vehicle to a certified Subaru dealership for diagnosis and repair — ideally with them covering the cost of work. Don’t accept an O2 sensor replacement without checking fuel trims, vacuum integrity, and other diagnostics. A proper scan and inspection are essential before any part is replaced.

The salesman called me unexpectedly — they’ve already completed the repair, even though I told them not to work on it until I approved. The total bill is $700 for a strut mount, alignment, and an O2 sensor replacement. This was unauthorized work and raises serious concerns about transparency.

It's true that Subaru vehicles are known for higher repair costs — common issues include strut replacements and headgasket failures. That said, the cost of repairs should be transparent and agreed upon upfront.

While Subarus can be expensive to maintain, a reputable mechanic quoted me $175 for a strut replacement and alignment. A Denso OEM O2 sensor costs around $82 on Amazon — I could replace it myself. The dealer promised a free diagnosis and quote after selling the car two days ago, yet now wants to charge $700 for work I didn’t authorize. This seems unreasonable. My concern remains: even if the O2 sensor is replaced, the P0174 code may return within a day or two due to an underlying vacuum leak — meaning the issue wasn't truly resolved.

While it's understandable that used cars carry risks, many extended warranties are not worth the cost. For those with experience in auto repair or access to trusted mechanics, saving on warranty fees and using their own expertise is often more practical. Be cautious of online retailers like Amazon — low prices may come with hidden shipping costs, damaged parts, incorrect inventory, or poor warranties. If a dealer performs repairs without your consent, stop payment on the credit card immediately. Unauthorized work is illegal; they should either remove the parts or negotiate a fair price.

I spoke with the manager today and he agreed to cover all costs — no charge to me! Thank you for everyone’s advice — it really helped clarify my next steps.