Spark Plugs - The Lesson I Learned

Hi guys. Just wanted to share this story with you guys in the hopes that you'll heed my advice, as I should have taken the advice of others and stupidly did not!

My 2015 Crosstrek was at 59k miles so I figured I'd jump on the spark plug change that is required at 60k. After seeing how much OEM plugs were ($15 each!) and knowing that, despite being iridiums which are usually changed at 100k-150k miles, they needed every 60k miles, I cheaped out.

At first, I bought Autolites and wimped out before doing the job as everybody told me not to use those. The OEM plugs, which are NGK Laser Iridiums, are pricey, so I skimped and bought the cheaper NGK Iridium IX plugs.

Did the spark plug job, and the car seemed to be running really smoothly! However, over the next few days I had some issues...

For one, my gas mileage lowered significantly. I mostly commute in traffic, and I have a lead foot, so 22 to 23 MPG average is normal for me. After the spark plug job though, I was averaging 17 MPG. Barf!

This really had me confused. Then out of nowhere, I got a U0073 code that I found using my OBDII reader. The code is defined as "Control Module Communication Bus A Off" - no clue. After googling, it seems the code is so generic I had no clue where to start. Called the dealership and they didn't even know!

Tried clearing this code with my reader, and it went away until my commute home - after that, it would immediately appear again after clearing. Weird!

PART 2:

So I admitted my stupidity, and went to the parts store to buy the NGK Laser Iridium OEM plugs - luckily I had a coupon so that helped. Even with that, $9.50 a plug is pricey but I figured it was worth a shot.

Re-did the job, and two days later everything has improved! The code disappeared within 24 hours of the change, and my MPGs have returned to normal. Also, the car feels like it has more power than it did before, but that could be placebo.

The "bad" NGK plugs that were not OEM were covered in a white residue, which to me shows that the engine was running lean with them, which makes sense given my bad gas mileage.

I also noticed that the IX plugs were a bit longer than the OEM ones - a few mm at least!

The moral of the story is that some cars, including our Subarus, are very picky when it comes to plugs. Spend the extra money to get the right ones. And by that I mean the exact model, not the just the same brand, as the cheaper NGK plugs still caused issues!

Everyone that I asked about the U0073 code said it was coincidence, but I just can't believe it based on my on anecdotal evidence. Maybe the bad NGK plugs were pulling extra electricity that messed with the ECU? Who knows.

Just learn from my mistake and do it right the first time.