I know the answer is likely yes, but really, I'm hoping to understand more about my options. Also, if I do schedule the diagnostic, I want to prepare myself so I can ask the right questions and ensure they run the right tests.
Sorry for the long post, but hopefully the detail is helpful.


Context:
Mazda 3 - 2017, Sport, 4dr Hatchback
110k miles

Early last week, my check engine light came on, so I took it to AutoZone, and they did a free scan with their OBD II scanner. It came back with:
P0303 - Cylinder 3 Misfire Detected.
The printout recommended replacing one ignition coil.

I bought and installed this ignition coil from AutoZone:
C1837 Duralast Ignition Coil
At the time, I didn’t realize it might be better to buy an OEM ignition coil or one from more trusted brands like NGK or Denso (which are coming up in my research now).

It still seemed like it was starting rough or misfiring on start-up, so I ended up buying NGK spark plugs:
NGK Laser Iridium Spark Plug - 94124, ILKAR7L11

However, after replacing them, it didn't seem back to normal yet. When I went to start it, it sounded fine at first, then seemed to drop out a bit just before starting.
I did notice that it was better once I started it a second time or after it would run for a bit.

Ultimately, the check engine light stayed on, so I took it to a local mechanic. They didn’t have time to do a full diagnostic but came out as a courtesy to hear it start. They heard what I was describing and cleared the check engine light. They did this so I could see if the check engine light would come back on.


Currently / The Question:
My car sounds a little better now when I start it. I’ve been driving it for a couple of days, and the check engine light has not come back on (I've driven a couple hundred miles on the freeway and some amount around my city).

I’m wondering:
- Should I still take it in as-is?
- Should I buy more ignition coils and replace them all first? (e.g., replace the other three and keep the Duralast, or replace all four with OEM/NGK/Denso/other brands?)
- Should I wait until the check engine light comes back on?

The cost is $225 per hour for diagnostics, so if I go, I want to ensure they run the right tests and that I know what to ask for.

Generally, I’m interested in having my car last as long as possible and run as well as it can. So I’m tempted to bring it in and have them check it out, but I’m unsure what specific services or tests to request. Also, I’m not sure how much value they can add if the check engine light isn’t on.

In another thread, I read about using an oscilloscope to analyze the ignition system, which sounds interesting.

What would you recommend to ensure my car is running well without over-investing in it?

I’m also open to suggestions for tests, fluid replacements, or maintenance ideas unrelated to this specific issue if you think they’d be helpful.

 

Welcome to the forum

 

Ok so lightly reading your detailed response let me say as I often do on many forums...
NEV ER rep[lace any part when you have a problem and a engine check light before you first completely confirm that the part has failed or directly the cause of the problem "failing". Doing so often will create additional problems unrelated to the first inresolved issu.

Ok moving on.....

Some Auto Parts and service shops etc., that offer a FREE scan only do a basic scan and not a detailed or even a complete diagnostic scan of the ECU. So as I read your response, they GUESSED based on their limited accessed information in your ECU.

You have a few possible things going on. My suggestion is finding an independent service shop that is ASE rated so you know they will have at least 1 ASE certified mechanic or better Diagnostic Technician employed. Pay the basic hour fee for a proper diagnostic of your Mazda. . You can go to a Mazda dealership, but you will pay more, and you will not always get a factory trained Mazda certified Auto mechanic working on your Mazda.

YES an oscilloscope to analyze the ignition system as well separate parts and even fueling and other systems as well. That other forums may have been me?? I have posted that information for a few decades and hundreds of times on various platform forms, car, trucks, boats, planes. motorcycle etc..LOL Scanners are great but so are scopes and combined usage of both insures finding the problem the first time no guessing!

 

Callisto, thanks very much for your response! Yes, I think you are correct that it was you who mentioned the oscilloscope

This is exactly what I was looking for, all of your info is helpful. I had not heard about ASE certified mechanics before and I'm now looking them up in my area.
(also helpful to know that I might not even get a Mazda-trained mechanic if I went to the dealer)

I'll circle back once I have a chance to take it in

 

Many don’t understand that a dealership is an independently owned business. The owners in several capacities contract with the auto manufacturer to sell their line. So while there are requirements most auto manufacturer do not required having a factory trained mechanic working in their service shop.
Example … 1 local Mazda contracts from time to time my service as a 30+ year and current ASE Master Technician L1 L2 ( L2 =diesel certified but I rarely almost never advise or work on them they smell and sound funny lol) to perform so service work . Everything except safety Recalls. In which case the owner must go to a dealerships with only a factory trained mechanic to have the safety recall performed.

In most all states any independent service shop that advertises or has signage on their shop ASE must qualify to use ASE if they have at least 1 Current certified ASE in any campsite except parts certification.

Please do respond your resolve it helps member and visitors when they can relate to a problem and see how others get it taken care of .

ASE

 

Quick update
The check engine light came back on, and after a few days of driving, it was starting even worse and almost stalled out on me today (it felt clunky and stuttered on the road)
When that happened, I pulled over, turned it off, and restarted the car. That worked well enough for me to make it to a shop.

The mechanic ran a diagnostic and this is what they came back with:

"Checked inside cylinders, and found the top of cylinder three to have a much larger portion having been "washed" by fuel. In the second to last photo under "Malfunction Indicator Lights" section, a minimal ring pattern can be seen to have been cleaned off top of piston, which is the same wash pattern from all three cylinders that are firing properly. In the last photo, there is a much larger portion of the top of the piston that has been washed. This indicates the fuel injector in cylinder three is leaking fuel into the cylinder. This explains the symptoms of rough cold starts - fuel builds up in cylinder three after engine is shut off, then upon starting, engine runs rough until excess fuel in cylinder three burns off. Intake has to come off to access fuel injectors, so an intake gasket is needed as well."

"Relating to misfire, found codes P0303 - Cylinder 3 misfire and P2308 - Ignition coil C secondary circuit. Verified there is an active misfire. Verified correct spark plugs and one ignition coil were installed. Verified swapping plug and/or coil from one cylinder to another did not move misfire, indicating plug/coil are not the issue. When disconnecting the electrical connector from #3 coil while engine is running, verified a heavy misfire presents, indicating the coil is firing. Tested and verified coil has power and ground. Verified that all four cylinders have correct compression."

"Replace cracked serpentine belt. Advise to replace stretch fit water pump belt as well. It is not cracked but appears to be original. Serpentine belt has to be removed to access water pump belt."

In total, they quoted me:
$2,296.87 - Replace all 4 fuel injectors.
$311.34 - BG Platinum Fuel Injection Service
$260.70 - Replace serpentine drive belt and stretch fit water pump belt.

I'm not exactly sure what to make of all that but that's what I've got so far
A lot of photos but this is what they sent me (minus the two that show the error codes mentioned above because I am limited to 20 photos)

 

First I want to say GREAT info in your last response. I wish more would post like that.
I looked at the values and most all are within range and those that are slightly close to border line are what I would say normal for the miles on the engine.
I am not sure I agree with a couple of how the mechanic tested parts but that also seems a normal thing in some shops.
I am not sure what exactly the estimate is for. It is a little vague. Can you send me a pm with the detailed estimate? If you do, make sure it includes their hourly shop rate.
I am looking into that BG fuel service...not sure how they claim that works well on a DI engine? Thier theory is nice but the ECU wont like it? JOKE= an Italian Tune-up may work better?
I also like how they word installing a new water pump as a "chore"... Quote= "stretch fit water pump belt"

 

Thanks, Callisto! I'm finding your responses and this forum to be really helpful.

Given the cost, I took it to another shop today to get another set of eyes on it. That unfortunately closed out the estimate so I can't see the detail anymore.
However, I know that the labor and parts were separate line items and added up to the numbers I mentioned above.
I do still have access to the inspection results so I'll see if I can send you that.

Yeah I wasn't so sure about that BG fuel service. I watched a video on youtube and wasn't convinced of the value. It seems like if I really need to remove the carbon buildup then I would have to walnut blast it (but I likely won't do that)
(video I'm referencing: Does BG Platinum GDI Intake Cleaner Remove Carbon? It's time to find out! - by: Technician Red)

I forgot to mention, the first diagnostic came out to $227.90 after tax (Durham, NC)
I was surprised that it was as much as the dealer but they were also the only place that had availability that day.

The second shop quoted me $110 for an hour of diagnostic. We'll see what they find when they check it out next week.

All in all, I was reasonably happy with the first shop but I'm hoping that a second opinion can give me more information to go off of

 

Diagnostic is generally a 1 hour labor charge. We are at 150-175 currently per hour which is in line with most other shops but lower than some shops in other areas of the state. CA>
However unless you make sure you get a currently certified Factory trained Mazda Auto mechanic or an ASE Certified Automotive Diagnostic Technician it a crap shoot to get assured the best diagnostic service and recommendations.

Personally pass on the F/I service you can get almost the same results in a couple or more or routine use of a bottle of Fuel treatment STP, Chevron Fuel treatment or a QT of Xylene if it is still allowed to purchase at hardware stores in your state. I have to travel 2 hours to RENO Neveda to but a case every 6-8 months. lol.
As for $260.70 - Replace serpentine drive belt and stretch fit water pump belt. well I suppose it depends on what brand parts they are useing it a little high ??

Now the fuel injection replacement cost WOW!!! What are they balanced performance fuel injectors? LOL

 

Okay, 10 days later and I have an update

The second shop is finished with their diagnostic and they are saying I need to replace the high-pressure fuel pump.
They are not like the first shop in that they did not provide a written explanation or photos so it's a bit harder for me to relay what they said over the phone.
It does sound like they tried a variety of things and ran a good amount of tests.

In total it was $275 for the diagnostic ($110 * 2.5hrs)
& they are quoting me $1,250 to replace the fuel pump.

I asked about the fuel injectors (because that's what the first shop said) and they didn't think that was the issue.