When a Toyota 4Runner shows both P0304 (cylinder 4 misfire) and P0300 (random multiple misfire) simultaneously, the combination almost always points to a failed ignition coil on cylinder 4 that is severe enough to cause both a cylinder-specific misfire and overflow into the PCM’s random misfire detection threshold: and on the 4.0L 1GR-FE V6 4Runner, a coil-swap test in under 20 minutes will confirm or rule out the coil, leaving spark plug condition and fuel injector function as the next diagnostic steps.
The simultaneous presence of P0300 alongside P0304 is diagnostically significant because it tells you the cylinder 4 misfire is not occasional or minor: it is severe enough that the PCM’s random misfire counter is also filling.
“My 2006 4Runner 4.0 started bucking at highway speed with P0300 and P0304 both on. Pulled the coil off cylinder 4 and swapped it to cylinder 6. Cleared codes and drove 20 miles. P0306 came back instead of P0304. Bought an OEM Denso coil, swapped it in, and both codes were gone. Total cost: $58 and an hour of my afternoon. The Toyota 1GR-FE is a great engine: you just have to keep up with the coils past 100k.”: Sam Nguyen, 2006 Toyota 4Runner Sport V6 owner
Why P0300 and P0304 Appear Together on the 4Runner
How the PCM Tracks Random vs. Cylinder-Specific Misfires
The PCM uses two separate misfire counters: Cylinder-specific counter: tracks misfire events in each cylinder independently. When cylinder 4 exceeds its threshold, P0304 stores. Random misfire counter: tracks total misfire events across all cylinders combined. When the total misfire rate exceeds a higher threshold, P0300 stores as well. When a single cylinder has a complete coil failure (not firing at all or firing intermittently), its contribution to the random misfire counter is high enough to trigger P0300 in addition to the cylinder-specific P0304. This is why P0300 plus P0304 together is more urgent than P0304 alone.
The 4.0L 1GR-FE Ignition System Layout
The 1GR-FE V6 uses six individual coil-on-plug (COP) assemblies. On the 4Runner, cylinder numbering is:
- Bank 1 (driver side on most configurations): cylinders 1, 3, 5
- Bank 2 (passenger side): cylinders 2, 4, 6 Cylinder 4 is the front cylinder on the passenger-side bank. The coil is accessible from above without any special tools beyond a 10mm socket to remove the coil hold-down bolt.
The Coil Swap Test on the Toyota 4Runner 1GR-FE
Step-by-Step Procedure
All six coil assemblies on the 1GR-FE are identical and interchangeable:
- With the engine cold, locate cylinder 4 (passenger bank, front position)
- Remove the ignition coil using a 10mm socket: one bolt holds the coil in place, then it pulls straight up
- Swap the cylinder 4 coil to cylinder 6 (passenger bank, rear)
- Install the cylinder 6 coil into cylinder 4’s position
- Clear all stored codes using a scan tool or by disconnecting the battery for 60 seconds
- Start the engine and drive through varied conditions including highway speed where the original misfire was most prominent Result interpretation:
- P0306 returns (instead of P0304): the misfire followed the coil to cylinder 6. The coil that was originally on cylinder 4 is the failed unit. Replace it.
- P0304 returns (unchanged): the fault stayed at cylinder 4 regardless of which coil is installed. Proceed to spark plug inspection.
- Both P0300 and P030X return: confirms the misfire is real, not an intermittent sensor issue.
Beyond the Coil: Other P0304 Causes on the 4Runner
Spark Plug Inspection on Cylinder 4
What to Look For on the 4Runner 1GR-FE Plug
If the coil swap test proves the coil is fine (misfire stays at cylinder 4 with two different coils), remove and inspect the cylinder 4 spark plug:
- Oil fouling (wet, dark, oily): valve stem seal wear or ring blow-by on that cylinder; compression test next
- Carbon fouling (dry, black, sooty): rich running on that cylinder; check injector
- White/pale insulator with light electrode: coolant burning in cylinder 4; combustion gas test on coolant
- Normal wear with excessive gap: replace the plug; standard wear resolution Spark plug specification for 4.0L 1GR-FE: Denso SK20HR11 or NGK ILFR6A. Torque: 18 ft-lb. Replacement interval: 60,000 miles for iridium.
Fuel Injector Fault on Cylinder 4
A clogged or partially failed injector on cylinder 4 provides insufficient fuel, causing a lean misfire that stores P0304. Injector diagnosis requires either a flow test at a shop or an injector balance test using a scan tool that can command individual injector activations. Injector replacement cost on 1GR-FE: $100 to $250 per injector. The 1GR-FE uses port injection (not GDI), so carbon buildup on the intake valve is not a factor: injector flow testing is more straightforward on this engine.
Compression Test for P0304 That Resists All Other Diagnoses
If coil, plug, and injector tests are all satisfactory but P0304 persists, a compression test is the final step before assuming an internal engine issue: 1GR-FE compression specification: 178 PSI (12.3 bar) standard, with no more than 14 PSI variation between cylinders. A cylinder 4 reading below 150 PSI or more than 20 PSI lower than the highest cylinder indicates internal wear.
| Diagnosis Step | What It Confirms | Cost if This Is the Fix |
|---|---|---|
| Coil swap test | Coil failure | $50 to $100 (OEM Denso coil) |
| Spark plug inspection and replacement | Plug wear or fouling | $60 to $100 (set of 6) |
| Injector flow test and replacement | Injector fault | $200 to $450 |
| Compression test | Internal engine condition | Diagnostic cost: $80 to $150 |
Frequently Asked Questions
What Causes P0304 and P0300 Together on a Toyota 4Runner?
The combination of P0304 and P0300 on the 4Runner most commonly means the cylinder 4 ignition coil has failed severely enough that its misfire contribution triggers both the cylinder-specific threshold and the overall random misfire threshold. A complete coil failure rather than partial failure is typical when both codes appear together. The coil swap test confirms or rules out the coil in under 30 minutes.
How Do I Know if It’s the Coil or Spark Plug Causing P0304?
Perform the coil swap test first. If the misfire code moves to the new cylinder position (P0306 instead of P0304), the coil is confirmed as the cause. If P0304 stays at cylinder 4 regardless of which coil is installed, pull the cylinder 4 spark plug and inspect its condition for fouling, unusual deposits, or excessive electrode wear.
How Much Does It Cost to Fix P0304 on a Toyota 4Runner?
An OEM Denso or NGK replacement coil: $40 to $80 per coil. A full set of six coils on a high-mileage 4Runner: $240 to $480. Six spark plugs replaced simultaneously: $60 to $100 in parts plus labor. Complete coil and plug service at an independent shop: $250 to $500 depending on how many coils are replaced.
The Bottom Line
P0300 and P0304 together on the Toyota 4Runner 1GR-FE is a coil failure in almost every case. Sam Nguyen’s experience is representative: the coil swap test is free, conclusive, and takes 20 minutes. Do it before buying a single part. If the misfire follows the coil, replace only the confirmed failed unit (or the full set if mileage justifies it). If the misfire stays at cylinder 4 through two different coils, the spark plug and injector become the next targets, in that order.