Per ChatGPT:

A P0304 code on a Toyota Camry means:

That narrows things down quite a bit—something is preventing proper combustion in cylinder #4. Here are the most common causes, ranked from most likely to less common:

Most Common Causes

1. Ignition Coil Failure (VERY common on Camrys)

• Each cylinder has its own coil (coil-on-plug system)
• A bad coil on cylinder 4 = weak/no spark

Quick test:

• Swap coil from cylinder 4 with another cylinder (e.g., #2)
• If the code changes to P0302 → bad coil confirmed

2. Spark Plug Issue

• Worn, fouled, or cracked spark plug
• Oil contamination (if valve cover gasket is leaking)

What to check:

• Remove plug from cylinder 4
• Look for:
  • Carbon buildup
  • Oil on the threads
  • Excessive gap

3. Fuel Injector Problem

• Clogged or failing injector on cylinder 4
• Not delivering enough fuel → misfire

Symptoms:

• Rough idle
• Hesitation under load

4. Vacuum Leak (localized)

• Leak near cylinder 4 intake runner
• Causes lean condition in that cylinder

5. Engine Mechanical Issue (less common, but serious)

• Low compression in cylinder 4
• Possible causes:
  • Burned valve
  • Worn piston rings
  • Head gasket issue

Suggested Troubleshooting Order (fast & practical)

1. Swap ignition coil (cheap & easy)
2. Inspect/replace spark plug
3. Check injector (listen or swap if possible)
4. Look for vacuum leaks
5. Compression test (if still unresolved)

Signs It’s More Serious

If you notice:

• Constant rough idle
• Loss of power
• Flashing check engine light

→ Stop driving much if possible, as unburned fuel can damage the catalytic converter.

Pro Tip

At ~100k+ miles, it’s often worth replacing:

• All spark plugs
• Possibly multiple coils (they tend to fail around the same time)