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F-150 EcoBoost P0300 Misfire: Diagnosis Decision Tree

Q: Can I keep driving my F-150 with a P0300 code?

If the light is steady and the truck runs acceptably, short gentle trips to a shop are reasonable — avoid boost and towing. If the light is flashing, no: unburned fuel is reaching the catalytic converter and can destroy it within one hard drive.

Q: Why does my EcoBoost misfire only under boost?

Boost raises cylinder pressure, which makes the spark plug's job harder. If the plug gap has worn open or a coil is weak, the spark fails exactly when combustion pressure is highest — so the miss shows up on highway merges and towing, not at a light idle.

Q: Should I replace all six coils or just the bad one?

If the failed coil is an outlier and the others test within specification, one is fine. On a high-kilometre truck with original coils, a full set is often the cheaper path over the remaining life of the vehicle. Ask for the test readings and decide on evidence, not on a blanket recommendation.

Q: How much does the whole repair usually cost in Canada?

Plugs alone at an independent shop typically land in the $300–$550 range; plugs plus a full set of coils more often run $800–$1,400, with dealerships sitting 30–60% higher. Diagnosis is usually a separate $120–$220 line at independents.

Q: Will a misfire fix itself if I clear the code?

No. Clearing the code erases the data the technician needs and the misfire monitor will set it again — usually within one drive cycle if the fault is real. Clear codes only after the repair, as a verification step.

2026-09-23 · 7 min read · Mecano Content Team

F-150 3.5L EcoBoost Misfire? A Step-by-Step Decision Tree for P0300 Codes (2015–2020)

Picture this. You merge onto the 401, roll into the throttle, and your 2015–2020 F-150 3.5L EcoBoost stumbles instead of pulling. Idle’s rough. Power’s gone. Check-engine light’s staring back at you. Here’s the thing: what you do next depends on which branch of this tree you’re on. So start at the top, find the line that matches your truck, and follow it down.

Step 1: Is the check-engine light flashing or steady?

Flashing light? Stop driving. That’s not a suggestion. A flashing light means the engine’s misfiring hard enough to torch the catalytic converter, and unburned fuel is hitting it right now. Pull over, call a tow. This is the one spot on this whole tree where paying for a tow beats paying for a $1,200–$3,000 converter.

Steady light? You’ve got some room. Drive gentle and get a diagnosis booked soon. No hard acceleration, no boost, and don’t let it sit there running rough for days. Want to know why these two lights get two different answers? Our guide to flashing vs. steady check-engine lights lays it out.

Step 2: Read the codes — which one you get decides the path

Got a scan tool, or had the parts store pull the code free? Good, that’s step one done. The code pattern tells you where to look next:

P0300 (random/multiple-cylinder misfire): the computer’s seeing misfires scattered across the engine, not stuck on one cylinder. On this truck, at highway mileage, worn spark plugs are the usual culprit — the gap opens up over time, and under boost the coil can’t jump it anymore. Head to Step 3.
P0301 through P0306 (one cylinder): the fault’s parked on a single cylinder. Plug worn on that cylinder? Replace the set. Plug looks fine? Suspect that cylinder’s coil — swap it to another cylinder and see if the code follows. Old trick, still works.
P0316 (misfire at startup, first 1,000 revolutions): expect a rough cold start that smooths out once it’s running. Plugs are still suspect number one, but a vacuum leak or low compression belongs on the list too — don’t skip the diagnosis and just guess.

These code definitions are the same on every vehicle — O’Reilly and KBB will tell you that much. What narrows it down is this engine’s own habits. Our engine misfire diagnosis guide walks through the full process a shop actually uses to narrow it down.

Step 3: Know this engine’s usual suspects

The 2015–2020 3.5L EcoBoost is a coil-on-plug twin-turbo V6, and it’s got a well-earned reputation:

Spark plugs. Ford’s maintenance schedule says replace them around 60,000 miles (about 96,000 km). Plenty of owners move that up to 40,000–50,000 miles, because this engine is hard on plugs, plain and simple. The factory plug is a Motorcraft SP-534, later superseded by the SP-580 — an iridium plug gapped around 0.028–0.032 inches. Worn plugs on this engine misfire under boost, not at idle — highway merges, passing, towing — because that’s exactly when cylinder pressure is highest and the ignition system is weakest.
Ignition coils. 2015–2016 trucks run Motorcraft DG-549 coils; 2017–2020 switched to DG-585. Neither one is a scheduled wear item, but here’s the catch: a worn plug forces its coil to put out more voltage to fire, and that cooks the coil over time. That’s why the plug and the coil on that cylinder usually get replaced together.
Coil boots and carbon tracking. See a black burnt line on a coil boot? That’s carbon tracking — spark found itself a shortcut. Replace the boot or the coil right along with the plug, or the misfire’s coming right back.

Plugs recent, coils testing strong, compression checks out fine? Only then do you move further down the tree — fuel injectors, vacuum leaks, low compression. That’s real diagnostic work at that point, not parts-guessing.

Step 4: What each branch costs in Canada

Here’s what these jobs typically run in Canada as of 2025–2026, based on the sources linked below. Your actual quote depends on your shop, your region, and how many parts turn out bad — treat these as a ballpark, not a promise.

Repair path	Independent shop	Dealership	DIY (parts only)
Misfire diagnosis	$120–$220	$180–$300	Code scan often free; scan tool ~$40+
All six spark plugs, installed	$300–$550	$450–$750	~$70–$150 in plugs
One ignition coil, installed	$150–$400	$200–$450+	~$40–$150 per coil
Full set of six coils, installed	$800–$1,400	$1,200–$2,200+	~$250–$900 in parts

A few things that table doesn’t tell you. Ontario independents commonly charge $130–$220 an hour, near the top of the national range — so if you’re in the GTA, expect labour to lean high. Parts are the smaller line here: decent aftermarket coils run around $40–$80 each, and Motorcraft plugs are roughly $12–$29 at a Canadian Ford parts counter. Even a “replace everything” quote should still land well under $1,000 in parts. Want the full breakdown? Our plug and coil cost guides (plugs, coils) go deeper on the numbers, and our diagnostic fee guide explains what a paid diagnosis should actually include.

One coil, or all six? One coil died at low mileage? Replace it and the plug on that cylinder, that’s it. Truck’s past 150,000 km and still running the original coils? A lot of technicians will just do the full set at that point — you pay for parts once instead of paying for diagnosis and labour twice down the road. Either way, make the shop show you the test that condemned the coil, and get a quote from more than one place before signing off on a big number — you can pull competing quotes from GTA shops through Mecano.

Step 5: If it still misfires after plugs and coils

Same code, same cylinder, still there? You’re in mechanical or fuel territory now — compression test first, then injector balance testing. P0300 back across several cylinders? Go back to basics: a vacuum leak, a failing fuel pump, or bad fuel can all scatter misfires just like that. This is the point where throwing more parts at it costs you more than the diagnostic time you were trying to save. Either way, keep driving on an unresolved misfire and you’re gambling with the catalytic converter — converter replacement starts around $1,200 installed in Canada, several times the price of the ignition parts that caused it.

Frequently asked questions

Can I keep driving my F-150 with a P0300 code? Steady light and the truck’s driving okay? Short, gentle trips to a shop are fine — just stay off the boost and don’t tow anything. Flashing light? No. Unburned fuel’s already hitting the catalytic converter, and it can wreck it in one hard drive.

Why does my EcoBoost misfire only under boost? Boost cranks up cylinder pressure, and that makes the spark plug’s job harder. If the gap’s worn open or a coil’s gone weak, the spark quits right when combustion pressure is highest — so you feel it on highway merges and towing, not idling at a light.

Should I replace all six coils or just the bad one? If it’s just the one bad coil and the rest test within spec, replace the one and move on. High-kilometre truck with the original coils still in it? A full set is often the cheaper path over the life the truck has left. Ask for the actual test readings and decide from the evidence — don’t just take a blanket recommendation at face value.

How much does the whole repair usually cost in Canada? Plugs alone at an independent shop: usually $300–$550. Plugs plus a full set of coils: more like $800–$1,400. Dealerships run 30–60% higher across the board. Diagnosis is its own line, usually $120–$220 at an independent.

Will a misfire fix itself if I clear the code? No. Clearing the code just wipes out the data your technician needs, and the misfire monitor will set it right back — usually within one drive cycle if the fault’s actually there. Only clear the code after the repair, as a way to confirm it’s fixed.