mjb

Update. I replaced all four plugs at the same time and it went exactly as you'd anticipate. The process itself is a pain, but after completed it started right up and ran smooth. Learned - replacing just two plugs can cause serious issues, including misfires on the "new plug" cylinders. Idk how often this has ever come up, but it can now be said - replace all four plugs at the same time. Takeaway - replace plugs at recommended interval. There is a big enough difference between old and new plugs that the vehicle struggles to run when you mix them.

Got the NGK plugs direct from NGK. The order was fulfilled by "sparkplugs.com" but I ordered from the NGK website. They came in original packaging, full price, no indication of counterfeit. I pulled the new plugs and tested continuity and resistance. 4.6K-4.9K ohms between terminal and center electrode (NGK says it should be 5K). I then tested the two unused plugs in the box - hovering between 4.7K-5.1K. I then tested the old plugs (12 years old at this point). The best read I could get on both of them was 3.4k-3.6K. I checked for continuity between terminal and hex and outside electrode - none. Same with old plugs. I checked the gap on all 6 plugs (new and the two old ones that are now out). All the same. i.e. - seems the new plugs are fine, not cracked or out of spec. I then put the old plugs back in and gave it a try. It took about 5 seconds to start. (five seconds holding the key in the start position). But when it started, it immediately ran smooth with no codes. I think the issue genuinely is caused by only replacing two of the plugs. Had I replaced all four plugs, I would have pulled the battery and had it out long enough for the ECU to "reset". This would have resulted in a difficult/rough start and made the engine search for idle again, but that happens every time you pull or replace the battery. In my old 5MT Outback, it would routinely rough idle, lack power, and even die when you stopped for the first 5-10 minutes after pulling or replacing the battery. Subaur says this is normal for the ECU to reprogram/adjust. I think just doing two plugs, with two new and two 12-year-old plugs may have been too much for the ECU to compensate or "adjust" one side compared the other. The real answer will come when I replace all four plugs with the new ones. Will be back when I've done that.

Looking for help on the "spark plug replacement" thread. Just replaced the 1 & 3 plugs on my 2011 Legacy GT. Did not get to the 2/4 plugs due to time. Car only has 69,000 miles. I think this is the first time they've been replaced. Used NGK OE plugs. Coil packs looked good. Didn't even unplug them to move them aside, just swapped the plugs and put everything back. Immediate rough idle and obvious misfire when I started the car. After a minute or two, the check engine light finally came on and showed P301 and P303 (misfire on cylinders 1/3). Any ideas for troubleshooting? Seems like the coil packs are rarely the issue, and the car was running 100% fine prior to changing the plugs. Is it possible that just changing one side caused an issue when they are all the same OE spark plugs?