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Worn spark plugs and failing ignition coils cause more Dodge misfire codes than everything else combined — and the platform has a famous quirk: the 5.7L HEMI runs two plugs per cylinder, sixteen in total, on a service interval far shorter than most owners realize (~30,000 miles for the factory copper plugs). The parts are cheap, the ten-minute coil-swap test names the guilty coil for free, and the whole job is honest driveway work. This guide covers what to buy, what to skip, and the HEMI-specific traps.
How Dodge Ignition Is Laid Out
- 5.7L HEMI V8 (Charger, Challenger, RAM, Durango): twin-plug heads — two spark plugs per cylinder, 16 total, fired by 8 coils that each serve both plugs of one cylinder. Twin plugs smooth combustion in the big hemispherical chambers; they also double your plug bill and double the chance one worn plug starts a misfire.
- 3.6L Pentastar V6 (Charger, Durango, Journey, Grand Caravan): 6 coil-on-plug units, one per cylinder, long-life iridium plugs (~100,000-mile spec). Front bank is easy; the rear bank in transverse minivans tests your patience, not your skill.
- 2.4L and other fours (Dart, Caliber, Avenger): 4 coil-on-plug units, straightforward access.
All of them use coil-on-plug (COP) ignition: each cylinder has its own coil, which is why coils fail individually and announce themselves as single-cylinder misfire codes — P0301–P0308, where the last digit names the cylinder.
Symptoms of Worn Plugs or a Failing Coil
- Misfire codes — one cylinder (P030X) points at that cylinder’s coil or plugs; random multi-cylinder misfires (P0300) point at age-worn plugs across the board
- Rough idle, vibration at stoplights, smoothing out at speed
- Hesitation or stumble under load — worn plugs need more voltage exactly when the engine demands it
- Misfires worse in rain or humidity — the classic cracked coil boot arcing to ground
- Hard starting, worse fuel economy, sluggish throttle response
- Flashing check engine light — active misfire destroying the catalytic converter; stop driving
Find the Culprit: The Free Tests, In Order
- Read the codes. A single-cylinder code (say P0303) makes this surgical; pure P0300 with high mileage usually means the whole plug set is due. The cylinder-misfire guide maps code → cylinder per engine.
- Run the coil-swap test. Move the suspect cylinder’s coil to a neighbor, clear codes, drive. Code follows the coil → coil condemned, ten minutes, zero dollars. Code stays → the coil is innocent; look at plugs and deeper.
- Pull and read the plug(s). Rounded electrodes and a wide gap = worn out. Oil-wet = valve-cover seep into the well (fix the gasket or the new coil dies young). Black soot = rich running; white blisters = lean/hot — both are clues beyond ignition, handled in the lean-code guide.
- Inspect boots and wells. Carbon tracking (thin gray lines down the boot), cracks, or green corrosion on the spring = replace the boot or coil; a dab of dielectric grease on reassembly prevents the next arc-out.
- On a HEMI, remember there are two plugs per hole. A cylinder misfire with one healthy-looking plug isn’t cleared until you’ve seen its twin.
Buying Right: Plugs
- Match the manual, not the marketing. The HEMI’s factory fit is copper-core Champion plugs — cheap, excellent conductivity, but a ~30,000–32,000-mile interval. Pentastar and most fours spec long-life iridium (~100k). Buy exactly what your engine calls for: HEMI-spec Champion plugs or NGK/Mopar iridium for the rest.
- “Upgrades” disappoint on twin-plug HEMIs. Exotic plugs at triple the price routinely idle worse than the humble factory copper. The manual’s spec is the performance spec.
- Always a full set — 16 on a HEMI, never “just the bad one”; mixed-age plugs chase you with rolling misfires.
- Gap and torque to spec. Pre-gapped isn’t always correctly gapped — check. Torque matters in aluminum heads: snug per the spec sheet, cold engine only, and modern plated-shell plugs go in without anti-seize (it over-torques the reading and invites loosening).
Buying Right: Coils
- OEM-quality only: Mopar, NGK, Standard, or Denso. The $15-per-coil bargain multipack is this site’s most repeated villain — they arrive weak, die in heat soak, and waste your diagnosis.
- Replace the failed one, inspect the rest. One dead coil at 150k doesn’t mandate eight new ones — but cracked boots or a second weak performer on the swap test justify doing the set while you’re in there.
- New coil, old problem? If a fresh coil keeps dying on the same cylinder, something is killing it: oil in the well, a worn plug with a huge gap demanding excess voltage, or wiring. Fix the cause, not the symptom.
Dodge Plug & Coil Costs
Typical US prices:
DIY note: plugs and coils are the gateway drug of Dodge wrenching — a socket, an extension, a torque wrench, and patience. The HEMI’s 16 plugs are a long afternoon, not a hard one. Budget tip: the difference between DIY and shop price on a HEMI plug job buys the next two plug sets.
Maintenance: The Interval Nobody Reads
The factory copper plugs that make the HEMI run so well wear fast — the book says roughly every 30–32k. The most common “mystery misfire” on a 60k-mile RAM is simply original plugs at double their service life. Put it on the calendar with oil changes; your converter (and our P0420 guide traffic) will thank you.
An ignition fault that flashes the check engine light is actively dumping raw fuel into the exhaust and cooking the catalytic converter. Park it, run the coil-swap test, fix it — a $60 coil beats a $1,500 cat every single time.
FAQ: Dodge Spark Plugs and Ignition Coils
How many spark plugs does a 5.7 HEMI have?
Sixteen — two per cylinder. The HEMI’s hemispherical combustion chambers use twin plugs for smoother, more complete combustion, fired by eight coils that each serve both plugs of one cylinder. It doubles the parts count for a plug change and means a cylinder misfire isn’t fully diagnosed until both of that cylinder’s plugs have been inspected.
How often should spark plugs be changed on a Dodge?
It depends on the engine, and the HEMI surprises people: its factory copper-core plugs are rated for roughly 30,000–32,000 miles, far shorter than the 100,000-mile iridium plugs in the 3.6L Pentastar and most four-cylinders. Original plugs still in a 60,000-mile HEMI are the single most common cause of its “mystery” misfires. Check the maintenance schedule for your exact engine and treat the HEMI interval like an oil-change appointment.
Can I replace just one ignition coil on my Dodge?
Yes — coils fail individually, and replacing the one the coil-swap test condemned is completely legitimate. Use the visit wisely though: inspect the remaining boots for cracks and carbon tracking, check the plugs’ age, and fix any oil seeping into the plug wells, because oil and worn wide-gapped plugs are what kill coils early. A second weak coil on the swap test is a fair argument for doing the set.
Are iridium plugs an upgrade for a 5.7 HEMI?
Usually not. The twin-plug HEMI was engineered around inexpensive copper-core plugs, which conduct better than precious-metal designs — the trade-off is the short 30k interval. Premium iridium “upgrades” at several times the price routinely deliver equal or worse idle quality in this engine. The dependable recipe is the manual’s exact spec, gapped and torqued correctly, replaced on schedule. Long-life plugs make sense where the factory specified them, like the Pentastar.
How much does it cost to replace spark plugs and coils on a Dodge?
A plug set runs $40–$150 in parts, or $80–$350 installed — the HEMI’s sixteen holes and minivan rear banks set the top of the range. A single ignition coil costs $100–$250 installed, with the part itself $50–$120 and the swap a minutes-long DIY. A full set of eight quality HEMI coils runs $400–$800+, which is rarely necessary at once — the free coil-swap test tells you exactly which one to buy.
Ignition work starts from a misfire code — the P0301–P0308 guide maps the code to the cylinder, and the P0300 guide handles the multi-cylinder case.