P0301 DTC, 210k mi. Still has Orig. plugs: Replace all Coils too?

This is embarrassing. I have never changed the plugs in my wife's 2013 Prius Plug-In (purch. new 10/23). Her last trip to do errands, it jerked aggressively she said, one time when she was accelerating normally (straight). I scanned for codes. My Autel MK 808S shows 4 Faults, all P0301 Cyl. 1 Misfire detected (1 each as Pending, Current, Permanent, and History. I was aghast when I checked my service log, and saw that I had never changed the spark plugs (I see NOW, due at 120k!). So, of course I will be changing the plugs.

Sounds like there is a decent chance that Cyl 1's coil may be bad. Chat GPT said after changing the plugs, swap Cyl. 1 & Cyl. 2's Coils, to see if the DTC moves to Cyl. 2. Good for troubleshooting the immediate failure, BUT.. Chat GPT mentioned that plugs with 200k+ miles, with worn wider gaps, "...causes Increased firing voltage demand on the coils. 120k–180k miles → common first failure. 200k+ miles → failure is not surprising at all. At 210,000 miles, original coils are officially on borrowed time. So yes — Cylinder 1’s coil being weak or failing is absolutely plausible.

So should I just bite the bullet and replace all four coils, as they may all be on their last legs by the higher current demand they have been working harder than normal to supply to jump the wider gaps?

Any particular Spark Plug brand(s) recommended, or brands to especially avoid?

I see coils going for between $35 to $75, or even $125 each for Denso. On the other hand, I also see
BDFHYK Ignition Coil Pack UF596 and Iridium Spark Plugs, set of 4 each for $47.69! #1 Best Seller, 4 stars, (4,300 reviews), 100+ bought in past month. That price seems WAY to cheap. Maybe those are shops buying, not DIY people putting them in their own cars??

Any recommendations for quality Coils at reasonable prices? Are 4 plugs + 4 Coils kits worth even considering? I have only seen no-name combo kits.

With the time involved to remove wipers, plastic and metal cowls, I want the replacement plugs and coil(s) if installed to last a long time. ;o)

 

Thanks for this. Today, I will remove the wipers and cowling and replace the plugs. I'll leave all that hardware off until it's driven and (fingers crossed), I am very lucky that solves the issue.

 

• I do not understand what you are saying. Can you clarify? I assume you are referring to the 12V starting battery. I did replace the original OEM 45 AH 12V AGM battery last year. I made some small physical adaptations to replaced it with a 55 AH AGM Deep Cycle battery. If interested in the details of how I did the small mods and lug conversion parts for this 12V battery conversion, see this thread:
‘Fitting mobility 12volt AGM battery’. Page 30, posts # 581, 583, and 585'.

• I have not been driving it when the severe shaking, intermittently, occurs. But my wife describes it as quite violent. That does not sound like a misfire. But if a small amount of coolant is leaking into Cyl. #1, maybe after the engine has warmed up and been shut off a few mins. (or longer), even during pauses from running when the HV battery takes over… That water in a cylinder would cause a much higher compression ratio in that cylinder making that piston not want to complete its compression stroke and make the engine ‘buck’. This worries me. [ I bought a new GMC work van in 1999. It was not driven that often, but in 2007 w/ 51k miles, it had hard starting issues. Then one day returning from work an hour from home, goes up a steep grade, it started bucking violently. I figured the engine had thrown a rod, but since it was still running, I kept going to the CHP weigh station at the top of the (Sunol) grade ¼ mile away. I parked it in the huge empty parking lot, with the trail of oil in its path. I then learned that GMC trucks of that era were known to have issues if coolant was not changed regularly. It wasn’t the mileage, but the 8 years, that made the known to be problematic coolant issue become catastrophic in this case. I had the 5.7L Vortec L31 replaced with the same long block crate engine for $8,700 (engine $5,700, labor $2,800, rodded radiator + oil cooler $125). ]

• Thanks for the warning. I’ll get some name brand, Iridium’s if not the same Denso plugs and do the plug swap today. (A prayer to the engine Gods. ;o)

 

Of course that was the first thing I checked, and added coolant...

BUT, I had NOT put 2 and 2 together until you asked this obvious question. THANK YOU!
Several days ago, this saga started with my wife's first report of the Check Engine light. The Engine Coolant was still showing in the plastic coolant reservoir, but way below the Min. level. I added 1/2 gal. (25% of 1.9 gal capac.) to bring it to full level.

JEEZ, with your reminder that the system had lost a half gal. of coolant, my brain just put 'loosing coolant' in the same thought as 'possible head gasket coolant leak'! In the past 12 years since new, the coolant reservoir has shown 'Full', with never having added an ounce of coolant.... Until now. So, for some amount of time in the recent past, it has lost 2 quarts of its 7.6 quart coolant capacity! There is no coolant residue in or around engine bay and no puddles leaks on the ground. I changed the engine oil and filter 4 months ago (Mobil 1 0-W20). At that time it was dirty, but not foamy or milky at all. Currently, the oil is clean and clear. No foaminess or milkiness. So coolant is not leaking into the oil.

I watched a ChrisFix YouTube “Do Head Gasket Sealers Actually Work (FULL 2yr TEST with Engine Teardown)” In it, he describes how to verify you have a head gasket leak into the combustion chamber using a Head Gasket Test Kit $38. Or Spill Proof Funnel $20.

Not sure I need a confirmation that the coolant is going into Cyl. #1. Is there anywhere else it could go? But I will order the Funnel kit to confirm exhaust bubbles coming out through the coolant.

I started watching some of this “COMPLETE 2010-2015 3rd gen Prius Head Gasket replacement "how-to" step by step Video. I think I could handle the job of replacing the head gasket. Though I could fork out $ ($5k to $7k+) to have a shop do the job, I’m not sure it would feel like a good investment. My wife loves this car and we’d love to keep it going. But paying for a shop would not feel great if the car would be worth only +/- $8,000 on the private sale market. So I guess I’d need to commit my time and bandwidth to doing the repair myself to make it feel like a worthwhile endeavor.

ETC(SS), once your question brought ‘head gasket’ et al replacement onto the radar, I spent the day in research, NOT doing the Spark Plugs replacement and Coil Swap test. ;o) Ultimately, the violent jerking my wife describes, does not sound at all like the car simply loosing power intermittently. P So I am transitioning my mind to the bigger job. Part of me likes the idea of proving to myself that, just turning 72, hey, I still have some skills (left), assuming it works out successfully, LOL.

 

I like that idea! It may just be yet further confirmation to see the top of a silver 'steam cleaned' piston in Cyl. #1, Ha, Ha.
The borescope mention makes me think... Did I buy one of those a few years back??... Or just research them thinking it would he cool and useful to have one. I'll have to poke around in my messy work bench, garage, and MIND, to see if anything comes to light! ;o)

After today's thinking that once I start into this project, I'm pretty sure, I'm in for the Whole Enchilada (Head Gasket and all parts down under there like: Water Pump, Timing chain tensioner, Thermostat, maybe 4 new coils...). If I'm investing 3-5 days (slower brain these days, yah know ;o), what's a few hundred more dollars here and there in parts 'while I'm in there' compared to MY time investment and the many thousands I'd pay in shop labor -and- marked up pricing on parts too, if I had to go that route.

 

I am shopping for a good borescope. I'll search PriusChat, Google, review Amazon, I did F Fwd thru Farm Projects' borescope review for borescope recommendations, especially for the smaller dia. 12mm Gen 3 Prius spark plug hole. A lot of variables to narrow down.

I saw your copy pasted questions over on my other thread where I answered this Q' plus a lt of new information: Head Gasket Replacement Shop Recommendations- San Francisco Bay Area I'll copy my answer and other details here, for continuity of this thread for future readers...
"The EGR has never been cleaned"

"I don't know over what period of time the Engine coolant level had dropped well below the minimum mark. It's my wife's daily driver, so I'm rarely under the hood to notice. I changed the oil in Sept of 2025. It's likely I would have noticed if the coolant reservoir was down much from the full mark at that time. After my wife first reported a check engine light 1/28/26, I saw that the coolant was WAY below the Min. mark, but still showing in the reservoir. Without running the engine, it took 1/2 gallon of 50/50 Zerex coolant to bring it to the full mark. It has remained there to date, but it has mostly been parked except one three mile errand after the coolant was added and the codes were cleared, thinking the issue would be solved by adding back the ≈ 25% of the missing coolant.

On her next, same 3 mile one-way errand, she got the shaking engine again. NOT from cold-start, but AFTER her short 5-10 min stop at the shopping center. She had one more (3rd) trip to that shopping center, with the same result of the jerking engine just after she started back to home. Tonight, I made that same trip, which was MY first time for a first-hand experience. It drove smooth as normal from cold-start on the 3 mile trip there. I was continuing my research, parked, and go figure two hours went by. ;o) When I put the car in Ready, it ran on the battery until I got onto the highway. As soon as I accelerated normally, it switched from HV battery to the engine, and immediately there was ≈ 2 seconds of engine shaking, then smooth again. As I continued this test drive, stopping several times for varying amounts of time (2, 5, 10 mins., every time the car switched to the ICE engine, the ICE rattled for 2-3 seconds, then smooth again. I had my Autel MK808S scanner in 'Live Mode' to monitor the ICE while I drove. Outside ambient temp. was ≈ 50ºF (10ºC). On my 3 mile drive out, the coolant temp was 158ºF (70ºC). On the return trip, the temp started rising 170, 185. So, as I was now in a 25 mph (40kph) zone, I slowed and accelerated strongly several times to see if it would continue to rise. It got up to 195ºF (90ºC). Hmmn? Then I continued driving, but normally, w/o the hard accelerations, and it settled at 170ºF (77ºC). I took it back out to the highway to see what a steady 65 mph (105kph) would get for temp. At that highway speed the coolant temp was a steady 195ºF (90ºC). Hmmn?

Would you agree that it seems to be a hot-soak restart misfire, not a cold start Misfire?
What does having a hot-soak restart misfire, instead of a cold start Misfire mean?"

Thank you for that great 'full engine section from repair manual.' !!

I did watch his three videos. They were very clear and meticulous. That is when I started thinking about not doing the head gasket job myself. After that questioning, I saw @rjparker 's post #2 on my Head Gasket Shop Rec'c thread mentioning the JDM engine swap option. I am liking the idea of THAT possible path big time now. I'm leaning towards paying a shop like Gasket Masters to do the engine swap job. I'm lucky that they have two shops relatively close to me. One in Stockton,CA- 45 mins. away. Haward, CA- 55 mins. away.

It just occurred to me, since I am seriously considering doing an engine swap, would it be unwise to do a stop-gap temporary fix by trying Bar's Leaks Head Gasket Fix? I'm not sure how much of a risk it would be of gumming up the cooling system any or somewhat, that would still be in place after an engine swap.

 

I intend to take our 2013 Prius to the Gasket Masters shop in Manteca CA, to get a full professional diagnosis. I still feel the desire to invest a bit of time to pull the wipers and two cowls to check the spark plugs before going to the shop. I just found out my brother-in-law has an inexpensive borescope that connects to your phone. So, now before I take it to the shop, I will be pulling and inspecting the plugs and peaking into the cylinders, especially Cyl. 1 that is getting the P0301 'Cyl. 1 Misfire Detected' code. I'll install 4 new plugs. The Car Care Nut recommends isolating the EGR valve/ System to eliminate it as an issue (4:00 min mark) and see if anything changes. I doubt the EGR System is at least the main issue for our Prius, but I'll be doing his test as due diligence. After the test, I will clean the EGR valve (carefully), its cooler and piping, because it has never been done.

rjparker recommends that I reinstall the two cowls and the wipers, rather than driving to the shop with the parts in the back (w/ nuts and bolts bagged and well labelled). More labor for me and more labor for the tech at Gasket Masters (that I pay them for ;o). When I talk to them, I may ask them their preference on this.