What P0304 actually means.
P0304 is the OBD-II code for "Cylinder 4 Misfire Detected." Same as P0301-P0303 in mechanics: the PCM tracks crankshaft RPM millisecond-by-millisecond and flags when cylinder 4's contribution is missing or weak.
The unique angle for P0304 is access. On a 4-cylinder engine, cylinder 4 is the last cylinder — opposite end from the timing belt/chain. On most modern transverse-mounted FWD cars (Toyota Camry 4-cyl, Honda Civic, Hyundai Sonata, Nissan Altima 2.5L), cyl 4 sits with its back against the firewall. There's often only inches of working room. Removing the cyl 4 coil-on-plug or spark plug requires creative angles, removal of intake plumbing, or in some cases removal of the upper intake manifold.
This means the same diagnostic that takes 30 minutes for P0301 (cyl 1, easy access) can take 60-90 minutes for P0304. The work isn't harder — it's just slower because you're reaching past obstacles.
Where is cylinder 4 on your engine?
Cylinder 4 location is more predictable than cyl 3 across engines:
Inline 4-cylinder engines: Cyl 4 is the rearmost cylinder — opposite end from the timing belt/chain. On transverse-mounted FWD: pressed against firewall. On longitudinal RWD: at the back of the engine (closer to the cabin firewall on most layouts). The single hardest cylinder to access on most inline-4s.
Toyota V6 (1MZ-FE, 2GR-FE): Numbering 1-3-5 / 2-4-6. Cyl 4 is the MIDDLE cylinder of Bank 2 (rear bank, against firewall). Bank 2 access is already worse than Bank 1 — middle of Bank 2 is among the hardest cylinders to reach on these engines.
Honda V6 (J35): Numbering 1-2-3 / 4-5-6. Cyl 4 is the FIRST cylinder of Bank 2 — start of the front bank (J35 puts Bank 2 in front, unusual). Better access than the Bank 1 cylinders on this engine.
Ford V8 (4.6L, 5.4L Triton, 5.0L Coyote): Numbering 1-2-3-4 on passenger side, 5-6-7-8 on driver. Cyl 4 is the rearmost cylinder of Bank 1 (passenger side, last). Hardest cyl on the Bank 1 side.
GM V8 (LS, Vortec): Numbering 1-3-5-7 driver / 2-4-6-8 passenger. Cyl 4 is the second cylinder of Bank 2 (passenger side, second from front). Reasonable access.
Chrysler/HEMI V8: Same 1-3-5-7 / 2-4-6-8 pattern. Cyl 4 is second on passenger side. Similar access to GM.
Common causes, ranked by probability.
From my shop log over 22 years. Cylinder 4 has one cause that's slightly elevated compared to cyls 1-3 because of its position at the end of the engine.
Diagnose it yourself in 45-60 minutes.
Same swap test methodology as P0301-P0303, but budget extra time for cyl 4 access on inline-4 engines.
Step 1 — Locate cylinder 4 (5 min)
Inline-4: rearmost cylinder, opposite end from timing. V-engines: see lookup table in §02. Don't guess — the wrong cylinder wastes hours.
On transverse inline-4: physically check the access situation before starting. You may need a long extension, swivel socket, or to remove the air intake assembly first.
Step 2 — Plan the access path (10 min)
This step is unique to P0304. On many transverse inline-4 engines, you'll need to remove components before reaching cyl 4 hardware. Common paths:
- Remove the engine cover (decorative plastic on top of engine)
- Remove the air intake duct connecting throttle body to airbox
- On some engines, remove the upper intake manifold plenum
- From below: sometimes accessible through wheel well after removing fender liner
Take photos before disconnecting anything. Snap multiple angles. You'll need them for reassembly.
Step 3 — Swap the ignition coil (15 min)
Once you have access, the swap test itself is the same as P0301: move cyl 4's coil to cyl 3, move cyl 3's coil to cyl 4. Reconnect electrical connectors. Make sure both seat firmly on the spark plugs.
Clear the code with your scanner. Drive normally for 5-10 miles, varying RPM and load.
Step 4 — Re-scan and interpret (3 min)
P0303 set: The coil is bad — it's now in cylinder 3's position. Replace it.
P0304 returned: Coil is fine. Move to step 5.
No code: Drive 50+ more miles to confirm. Some misfires are intermittent.
Step 5 — Swap the spark plug (15-20 min)
If coil swap didn't move the misfire, swap cyl 4's plug with cyl 3's. Same logic.
While the plug is out, inspect it visually. Compare to a known-healthy plug from cyl 1 or 2. Black/sooty = running rich. White/chalky = running lean. Wet with oil = oil burning. Wet with gasoline = no spark (coil dead — contradicts step 4 result).
Pay attention to the gap. If it's stretched beyond spec (typically 0.030"-0.050"), the plug is worn even if it looks OK.
Step 6 — Test injector + check PCV (10 min)
If swapping didn't fix it, test the cyl 4 injector electrically. Multimeter resistance check (12-17Ω for most modern injectors). Listen for click with stethoscope or screwdriver.
While you're at the rear of the engine, inspect the PCV valve and breather hoses. These often connect near cyl 4 on inline-4 engines. A cracked or disconnected PCV hose can cause unmetered air at cyl 4 specifically. Replace if damaged ($5-15).
Step 7 — Compression test if needed (15 min)
Last diagnostic step. Pull the cyl 4 spark plug, install compression gauge, crank for 5-7 cycles with throttle held wide open. Healthy: 150-200 PSI. If cyl 4 is 50+ PSI lower than cyls 1-3, there's internal damage.
Do a wet test if dry compression is low: squirt 1-2 teaspoons of clean engine oil into the spark plug hole, recrank, and read again. If compression rises significantly, the rings are worn. If it stays low, valves or head gasket are leaking.
Bench-test specs for the diagnostic-curious.
The electrical and mechanical specs are the same as P0301-P0303 — they're universal across cylinders. The unique value here is understanding cyl 4 access patterns.
Cylinder 4 location across common engines
Cylinder 4 location lookup table
| Engine Family | Cyl 4 Bank | Physical Location | Access Difficulty |
|---|---|---|---|
| Inline 4-cyl (transverse FWD) | Same as cyl 1 | Rearmost (firewall side) | Hard (worst on inline-4) |
| Inline 4-cyl (longitudinal RWD) | Same as cyl 1 | Rearmost (cabin side) | Medium (cabin side) |
| Toyota V6 (1MZ, 2GR, 3.5L) | Bank 2 (rear) | Middle of Bank 2 | Hard |
| Honda V6 (J35) | Bank 2 (front in J35) | First of Bank 2 | Easy |
| Ford V8 (4.6, 5.4, 5.0) | Bank 1 (passenger) | Rearmost passenger side | Medium |
| GM V8 (LS, Vortec) | Bank 2 (passenger) | Second from front, passenger | Easy |
| Chrysler HEMI / Pentastar | Bank 2 (passenger) | Second from front | Easy |
| BMW V8 (N62, S65) | Bank 1 (driver) | Rearmost driver side | Medium |
Ignition coil resistance specs
Same specs as P0301-P0303 — coil resistance is universal across cylinders.
| Coil Type | Primary Resistance | Secondary Resistance | Verdict |
|---|---|---|---|
| Standard COP | 0.4–1.5 Ω | 5–15 kΩ | Open or short = replace |
| Pencil COP (BMW, VW) | 0.5–1.0 Ω | N/A integrated | Replace as set if multi-fail |
| Coil pack (older 4-cyl) | 0.5–2.0 Ω | 8–18 kΩ | Tests two cylinders at once |
Compression test target values
| Cyl 4 Reading | vs Other Cylinders | Diagnosis | Action |
|---|---|---|---|
| 150-200 PSI | Within 10% | Healthy | Look at coil/plug/injector |
| 130-150 PSI | 10-20% lower | Aging engine | Wet test to refine |
| <100 PSI | 50+ PSI lower | Major mechanical fault | Burnt valve or HG — head work |
| 0 PSI | No compression | Catastrophic | Hole in piston, dropped valve |
Torque specifications
| Component | Torque (lb-ft) | Torque (Nm) | Notes |
|---|---|---|---|
| Spark plug (M14, gasketed) | 18–22 lb-ft | 25–30 Nm | Light anti-seize on threads |
| Coil-on-plug bolt | 7–10 lb-ft | 10–14 Nm | Plastic threads — easy to strip |
| Air intake assembly clamps | 25–30 in-lbs | 2.8–3.4 Nm | Often needs removal for cyl 4 access |
| Upper intake plenum (V-engine) | 7–9 lb-ft | 10–12 Nm | Star pattern, multiple passes |
| PCV valve / hose clamps | Hand-tight | N/A | Don't crush hoses |
Diagnostic procedure summary
- Verify cyl 4 location — Inline-4: rearmost. V-engines: lookup table.
- Plan access path — Photos before disassembly. Air intake removal common on inline-4.
- Swap test the coil — Move cyl 4 coil to cyl 3. Drive 5-10 miles.
- Swap test the spark plug — If coil swap didn't move it.
- Inspect PCV valve and hoses — Often near cyl 4 on inline-4 engines.
- Test injector electrically — Resistance + click test.
- Compression test — Final mechanical diagnostic.
What P0304 feels like.
| Symptom | How common |
|---|---|
| Rough idle | 85% |
| Loss of power, especially under load | 70% |
| Hesitation on acceleration | 55% |
| Vibration through firewall (inline-4) | 50% |
| Reduced fuel economy | 45% |
| Flashing CEL (severe) | 15% |
On inline-4 engines, P0304 vibration tends to feel different from P0301 misfire — you feel it more through the firewall and dash area because cyl 4 is right there. Cyl 1 misfires feel more through the front of the car. Subtle distinction but useful for confirming the affected cylinder.
Real cost breakdown.
Same parts costs as P0301-P0303, but inline-4 cyl 4 carries an access labor premium at most shops:
| Repair | Parts | Labor | DIY Cost | Shop Cost |
|---|---|---|---|---|
| Inline-4 cyl 4 coil replacement | $30–$100 | 30–60 min (access) | $30–$100 | $200–$400 |
| V6/V8 cyl 4 coil replacement | $30–$100 | 15–60 min (varies) | $30–$100 | $150–$350 |
| Single spark plug | $5–$20 | 15–45 min | $5–$20 | $80–$220 |
| PCV valve + hoses | $15–$40 | 15–30 min | $15–$40 | $80–$180 |
| Single fuel injector | $80–$200 | 1–2 hrs | $80–$200 | $300–$600 |
| Walnut blasting (GDI) | $40 (media) | 3–5 hrs | $300–$500 | $400–$700 |
| Compression issue (head work) | $200–$600 | 10–20 hrs | $300–$800 | $1,500–$4,000 |
The right order to actually fix it.
- Verify cyl 4 location ($0) — Use lookup table for your engine.
- Plan access path ($0) — Photos before disassembly on inline-4 engines.
- Swap the coil ($0) — Move cyl 4 coil to cyl 3. Drive 5-10 miles. Solves about 42% of cases.
- Swap the spark plug ($0) — If coil swap didn't move it. Solves another 26% — and watch for "neglected plug" pattern on cyl 4 specifically.
- Replace the bad component ($5-100) — Once swap test confirms which is failed.
- Inspect PCV valve and hoses ($0-15) — Often near cyl 4 on inline-4 engines.
- Test injector electrically ($0) — Resistance + click.
- Replace injector if bad ($80-200) — Match OEM or quality aftermarket.
- Compression test ($25) — Last diagnostic before head work.
- Internal engine work ($1,500+) — Last resort.
Can you keep driving?
- Solid CEL with P0304: 1-2 weeks of normal driving is OK. Avoid heavy throttle.
- Flashing CEL with P0304: Active misfires reaching the catalytic converter. The unburned fuel can melt the cat within a few miles. Do not drive — fix or tow.
- Inline-4 specific: Cyl 4 misfires can be slightly worse for the cat than cyl 1 because the unburned fuel from cyl 4 has the longest path through the exhaust manifold to reach the cat — more time for it to mix and ignite improperly.
P0304 patterns by brand.
| Brand | Most common cause | Notes |
|---|---|---|
| Toyota 4-cyl (2AR-FE, etc.) | Coil failure | Camry, RAV4 — cyl 4 firewall side. Coils sometimes ignored during plug service. |
| Honda 4-cyl (1.5T, 2.0L) | Coil failure | Civic, CR-V — air intake removal needed for cyl 4 access. 1.5T turbo engines run hot near firewall. |
| Toyota V6 (Camry, Sienna) | Bank 2 coil | Cyl 4 is middle of Bank 2 — hardest cyl on this engine. |
| Ford V8 (Triton 5.4L) | Coil failure | Cyl 4 is rear of Bank 1. Triton coils famously short-lived. |
| GM 5.3L Vortec | Coil failure | Cyl 4 is second on Bank 2 (passenger). Easy access. Replace coils as set if multi-aged. |
| VW/Audi 2.0T | Carbon-fouled valve | GDI engines. Walnut blasting common at 80-100k miles. |
| BMW 4-cyl (N20, B48) | Pencil coil failure | Replace as set when one fails. Pencil coils heat-sensitive at firewall side. |
| Subaru boxer | Spark plug | Boxer plug access is difficult on all cylinders — cyl 4 isn't notably worse than others. |
Questions people always ask about P0304.
Two real reasons: (1) heat exposure at the firewall side reduces airflow over the coil, accelerating heat-induced failures; (2) plug service neglect — when shops do partial plug changes, cyl 4 is the one most likely to be skipped. So a cyl 4 plug at 100k miles can be sitting next to cyl 1-3 plugs that are 50k newer.
Only if 100k+ miles AND already had multiple failures, OR if the labor to access cyl 4 is so high that you might as well do all 4 while you're in there. On a Honda Civic 1.5T where cyl 4 access takes 25 minutes and the other 3 take 5 minutes each, doing all 4 in one session at 100k+ is reasonable. At 60k miles, just do the bad one.
Sometimes, on transverse inline-4 engines. With the front wheel off and fender liner removed, you can sometimes reach the cyl 4 plug well from the wheel-well side. This avoids the air intake disassembly. Works on some Hondas and Toyotas — your mileage varies. Check before disassembling intake.
Yes, surprisingly often on inline-4 engines. The PCV valve and breather hoses are usually mounted near the rear of the engine — right next to cyl 4. A cracked PCV hose introduces a localized vacuum leak that affects cyl 4 specifically. Easy to overlook. Visual inspect the PCV system any time you have cyl 4 misfire.
Idle-only misfires often point to a vacuum leak (idle is when manifold vacuum is highest, magnifying small leaks) or a PCV issue. Try the swap test specifically at idle — let it idle for 5 minutes after clearing the code. If misfire stays even after swap, focus on PCV and vacuum leaks near cyl 4.
Yes — fuel injector cleaner is added to the gas tank and reaches all cylinders equally through the fuel rail. If cyl 4's injector is partially clogged, a bottle of Techron or BG 44K can dissolve mild deposits. Worth trying ($8) before replacing the injector ($80-200). Won't help with electrical injector failures.