Cylinder 3 Misfire Detected. The engine control module has counted repeated combustion failures in cylinder number 3, identified by a momentary loss of crankshaft acceleration at that cylinder's position in the firing order.
HighLimited driving$90–$4,200 typical repair
Quick answer
P0303 means cylinder 3 specifically is not firing reliably. The causes are the same short list as any single-cylinder misfire — that cylinder's coil, plug or injector — but the bill is not the same, because where cylinder 3 physically sits varies enormously by engine layout. On an inline-four it is a middle cylinder you can reach in minutes. On a Toyota or Nissan transverse V6 it is on the rear bank against the firewall, and the intake manifold has to come off to touch it, which turns a $150 job into a $500 one. Find out which case you are in before you accept a quote.
Diagnostic summary
Problem
P0303 — Cylinder 3 Misfire
Severity
HighNeeds attention promptly
Can you drive?
Limited driving
Steady light, mild shake: short local trips to get it repaired are reasonable. Flashing light: stop and have it towed. A cylinder that is not burning its charge sends raw fuel into the exhaust, and it lights off inside the catalytic converter instead — substrate damage can start within about fifteen minutes of highway running. The tow costs less than the converter.
Typical repair
$90–$4,200
System
engine
Common causes
Failed ignition coil on cylinder 3, Spark plug left in past its interval — especially on a rear bank, Restricted or failed fuel injector on cylinder 3
Symptoms drivers report
▸A rhythmic shake at idle, usually strongest with the transmission in Drive and your foot on the brake.
▸Hesitation or a flat spot when accelerating from a stop.
▸A flashing check engine light during the misfire, steady afterwards.
▸An uneven, puffing exhaust note at idle.
▸A smell of unburned gasoline at the tailpipe.
▸Fuel economy down noticeably — a quarter of a four-cylinder's power is missing, a sixth of a V6's.
What triggers this code
1
Failed ignition coil on cylinder 3
Very common
The coil sits directly on the plug, in sustained under-hood heat, and its secondary insulation eventually breaks down. Spark then leaks to ground instead of crossing the plug gap, and it fails under cylinder pressure first — which is why the misfire often shows up under load before it shows up at idle.
How to spot it
Swap cylinder 3's coil with another cylinder on the same bank. If the code moves with it, the coil is the fault.
The fix
Replace the coil and that cylinder's plug together. A coil that has been arcing usually leaves a fouled plug behind it.
$80–$400•DIY friendly
2
Spark plug left in past its interval — especially on a rear bank
Very common
Worn plugs need more voltage than an aging coil can supply. There is a specific version of this on transverse V6s: rear-bank plugs are behind the intake manifold, and a cheap plug service sometimes covers only the three cylinders that are easy to see. If cylinder 3 is on that rear bank, its plug may be tens of thousands of miles older than the receipt suggests.
How to spot it
The cylinder 3 plug's gap is well beyond specification, or its electrode is visibly more worn than plugs from the other bank.
The fix
Replace with the specified plug type and gap. If the intake is already off, do the whole bank — the labor is spent either way.
$40–$500•Moderate DIY
3
Restricted or failed fuel injector on cylinder 3
Common
A partially clogged injector delivers a poor spray pattern and too little fuel; an electrically failed one does not open at all. On direct-injection engines, carbon on the injector tip produces the same result. Either way the cylinder is short of fuel while its neighbours are fine.
How to spot it
Coil and plug swaps do not move the code. A stethoscope shows cylinder 3 ticking differently from the others, or its winding resistance is more than about 10% off the rest of the set.
The fix
Bench-clean and flow-test if it is only restricted; replace it if the winding has failed. New seals whenever an injector is disturbed.
$220–$750•Advanced DIY
4
Head gasket leaking between the middle cylinders
Occasional
On an inline-four, the narrowest and hottest part of the head gasket is the bridge between cylinders 2 and 3. It is the first place the fire ring gives up after an overheat, and coolant seeping into a cylinder both quenches combustion and kills the plug. This is why P0303 arriving alongside P0302 on a four-cylinder deserves a cooling-system test before anyone buys another coil.
How to spot it
P0302 and P0303 stored together, coolant disappearing with no visible leak, white sweet-smelling exhaust on a cold start, or a plug from cylinder 3 that comes out steam-cleaned white while the others are tan.
The fix
Block-test the coolant for combustion gases to confirm, then head gasket replacement with the head checked for flatness.
$1,200–$3,200•Shop recommended
5
Low compression in cylinder 3
Occasional
A burnt exhaust valve, a worn valve seat or broken rings mean the charge is never properly squeezed, and no amount of spark or fuel compensates. Carbon buildup holding a valve off its seat belongs in this group too, and is common on high-mileage direct-injection engines.
How to spot it
Compression in cylinder 3 reads more than 15% below its neighbours. Squirting oil down the plug hole raises the number if the rings are worn, and changes nothing if a valve is the problem.
The fix
Leak-down test to locate the escape path, then valve work or engine repair as the results dictate.
$900–$4,200•Shop recommended
How to diagnose it
1
Find out which bank cylinder 3 is on before spending anything
This single question decides whether you are looking at a half-hour job or a half-day one. On inline engines cylinder 3 is simply the third from the accessory-belt end. On V6s it depends entirely on the manufacturer's numbering: Toyota and Nissan put odd cylinders on bank 1, which on a transverse installation is the bank against the firewall, so cylinder 3 is buried. Honda numbers 1-2-3 straight across the front bank, so cylinder 3 is right in front of you. Two V6s of similar size, same code, very different repair.
Result: Sets your expectation for labor and tells you whether to do the job yourself.
2
Scan for companion codes, not just this one
P0303 on its own points at that cylinder's own hardware. P0303 with P0300 means a general misfire the computer sometimes attributes to cylinder 3, which widens the search to fuel pressure and timing. P0303 with P0302 on an inline-four points at the gasket bridge between the two middle bores. P0303 with P0171 suggests unmetered air reaching that cylinder rather than an ignition fault.
Tools: OBD-II scan tool
Result: Redirects the diagnosis before you spend money on the wrong branch.
3
Read the freeze frame
Record engine speed, load and coolant temperature at the moment the code set. Cold-idle-only misfires point at plugs and coils. Misfires only under load point at a coil breaking down against cylinder pressure, or at fuel delivery. Misfires only once the engine is hot raise the odds of a gasket or a valve.
Tools: OBD-II scan tool with freeze-frame support
Result: Tells you which operating condition to reproduce when testing.
4
Swap the coil within the same bank
Move cylinder 3's coil to another cylinder and watch where the code goes. On a V6, swap it with cylinder 1 or 5 rather than with a cylinder on the opposite bank — same-bank cylinders are already exposed once you have access, and you avoid a second round of disassembly. Clear the code, drive until the light returns, and rescan. If the new code names the cylinder you moved the coil to, the coil is bad.
Tools: Socket set, OBD-II scan tool
Result: Confirms or rules out the single most likely cause at no cost.
5
Pull the plug and read what it tells you
Compare cylinder 3's plug against one from a cylinder that is behaving. Tan is normal. Dry black soot means rich mixture or weak spark. Oil-wet means oil is getting past rings or valve seals. Fuel-wet means the injector works and the ignition does not. A chalky-white, steam-cleaned tip on an engine that has been losing coolant is the classic head gasket signature and should stop you buying ignition parts.
Tools: Spark plug socket, Extension and ratchet, Torque wrench
Result: Converts the code into physical evidence from inside the cylinder.
6
Compression, leak-down, or a coolant block test
With ignition and fuel eliminated, the cylinder is not sealing. Compression gives the number and leak-down gives the escape route — air from the tailpipe is an exhaust valve, from the intake an intake valve, from the oil filler the rings, and bubbles in the coolant a head gasket. If coolant has been disappearing, run the block test first: it detects combustion gases in the coolant in a few minutes and costs a fraction of a teardown.
Result: Pins down the mechanical failure before anyone quotes a head job.
Repairs & cost
Coil and plug — accessible cylinder
Any inline engine, or a V6 where cylinder 3 is on the front bank. One coil, one plug, dielectric grease on the boot, done in well under an hour. This is the outcome for the majority of P0303 repairs and it is firmly in DIY territory.
$110–$320DIY friendly0.5 hrs labor
Coil and plug — rear bank of a transverse V6
Same parts, very different job. The upper intake manifold, and sometimes the wiper cowl, has to come off to reach the rear bank, which is where the extra 1.5 to 3 hours of labor goes. This is the honest reason a shop quotes more for P0303 than for the identical failure on cylinder 2, and it is worth asking them to price the whole bank of plugs while they are in there.
$280–$700Moderate DIY2.5 hrs labor
Fuel injector cleaning or replacement
Ultrasonic cleaning with a before-and-after flow test runs $25 to $40 per injector and recovers many restricted ones. Replacement is the answer when the winding has failed. Direct-injection injectors cost several times more than port injectors and need a new high-pressure seal every time they are removed.
$220–$800Advanced DIY1.5 hrs labor
Head gasket or cylinder head repair
The expensive branch: gasket replacement with the head resurfaced and checked for cracks, or valve work when a seat has receded. Anything that let the gasket fail in the first place — a marginal cooling system, a stuck thermostat — has to be fixed at the same time or the new gasket follows the old one.
$1,200–$4,200Shop recommended9 hrs labor
Vehicles that set this code often
Design details that make some models more prone to this fault than others.
Toyota Sienna
On the 2GR-FE V6, bank 1 is the firewall side, so cylinder 3 is one of the three plugs hidden behind the intake manifold. Toyota's own procedure calls for removing the upper intake to service them, which is why a rear-bank coil job on a Sienna or Highlander is quoted at two to three hours rather than thirty minutes.
Nissan Maxima
VQ-series coils fail individually and often, and on the transverse VQ35 cylinder 3 sits on the rear bank. The combination is unpleasant: a cheap part behind expensive access. Owners who have already had one rear-bank coil fail frequently replace all three on that bank while the intake collector is off.
Honda Accord
The J-series V6 numbers cylinders 1-2-3 across the front bank, so a P0303 on a Honda V6 is the easy version of this code — the coil is visible from above with the hood open. Worth knowing before you accept a quote that assumes rear-bank labor.
Subaru Outback
On the flat-four, cylinders 1 and 3 share the passenger side, with 3 toward the rear of that bank. Access is between the engine and the frame rail rather than from above, and reaching it usually means going in through the wheel arch. Labor lands between the inline-four and the V6 rear-bank cases.
Where cylinder 3 sits, and why it moves the price
Cylinder numbering is a manufacturer's convention, not a description of position. For cylinder 3 that convention has direct financial consequences, because on half of the common V6 layouts it lands on the bank facing the firewall.
Layout
Where cylinder 3 usually sits
Access
Inline-4
Third from the accessory-belt end, second from the transmission
Straight down from above — easy
Toyota / Nissan transverse V6
Rear bank (odd cylinders on bank 1, bank 1 at the firewall)
Upper intake manifold off — 1.5–3 hours
Honda J-series V6
Front bank (Honda numbers 1-2-3 across the front)
Visible from above — easy
Longitudinal V6
Depends on maker; both banks reachable from the fenders
Moderate
Ford modular V8
Passenger bank, third cylinder back
Moderate — tight against the brake booster
GM small-block V8
Driver bank, second cylinder back
Moderate
Subaru flat-4
Passenger side, rear cylinder of that bank
From below or through the wheel arch
Typical location of cylinder 3 by engine layout
If cylinder 3 is buried, plan for one teardown
When the intake manifold has to come off, almost all of the cost is labor to get there. The parts are the cheap part. That changes the sensible strategy: on an accessible cylinder you replace one component at a time and retest, but on a buried one that approach means paying the access cost twice.
Do the whole bank's plugs, not just cylinder 3's. Three plugs cost $30 to $90 and you will never have cheaper access to them.
Inspect the intake gaskets while they are exposed — a rear-bank vacuum leak produces exactly this code and is invisible with the manifold in place.
Consider all three coils on that bank if they are original and past 100,000 miles. Coils bought at the same time fail at similar times.
Do a compression check on the bank before reassembly, while the plugs are already out. It costs almost nothing at that moment and rules out the expensive branch.
Replace the intake manifold gasket rather than reusing it, if the design uses a crush-type gasket.
The same logic applies to the timing of the repair. If the plugs on that bank are near their interval anyway, a P0303 is the moment to do the scheduled service, because you are paying for the access regardless.
Frequently asked questions
▸Why did my shop quote more for P0303 than my neighbour paid for P0301?
Almost certainly because cylinder 3 is on the rear bank of your engine and cylinder 1 was on the front bank of theirs. The coil and plug cost the same; the difference is one to three hours of labor removing the upper intake manifold to reach it. It is a legitimate charge, not an upsell — but ask them to confirm which bank the cylinder is on, and ask what it would cost to do all three plugs on that bank while it is apart.
▸Which cylinder is number 3 on my engine?
On an inline-four it is the third from the accessory-belt end. On V6s it depends on the maker: Toyota and Nissan put cylinders 1, 3 and 5 on bank 1, which faces the firewall on a transverse installation, while Honda's J-series numbers 1-2-3 straight across the front bank. Confirm from service information for your engine, or prove it by swapping a coil and seeing which cylinder the next code names.
▸I have P0302 and P0303 together on a four-cylinder. What does that mean?
Two adjacent middle cylinders misfiring on an inline-four is the pattern for a head gasket failing at the bridge between bores 2 and 3 — the thinnest and hottest section of the gasket. Before buying ignition parts, check whether coolant is disappearing and run a combustion block test on the coolant. It takes minutes and either confirms the expensive diagnosis or clears it. It is also worth reading up on car overheating, since that is usually what caused it.
▸Can I just replace the coil without testing anything?
On an easily accessible cylinder that is a defensible gamble — the part is cheap and it is the most likely cause. On a buried rear-bank cylinder it is a bad bet, because if you guess wrong you pay the access labor again. The coil swap test costs nothing and takes ten minutes, so do it before you commit money to a job with real labor attached.
▸Will P0303 fail an emissions test?
Yes. Misfire codes are emissions-related, and a stored code with the light on is an automatic failure at an OBD-II inspection. After the repair, drive enough for the readiness monitors to complete before you retest — clearing the code and going straight to the station fails on incomplete monitors instead.
▸Is P0303 more serious than a general misfire code?
It is more specific, which is better for you. P0300 means the computer detected misfires it could not attribute to one cylinder, so the search covers fuel pressure, timing and everything shared. P0303 hands you the location and narrows the likely causes to parts serving that one cylinder. Same urgency, much shorter list.