Toyota P0304: Cylinder 4 Misfire? Here’s How to Fix It Fast

That blinking check engine light got your stomach dropping? A Toyota P0304 code means your engine’s fourth cylinder stopped pulling its weight — and your car noticed. The good news? Most fixes are cheaper than you think, and many are DIY-friendly. Stick around because this guide walks you through every cause, every fix, and every dollar you might spend.

What Does Toyota P0304 Actually Mean?

The P0304 code means your engine control module (ECM) detected a misfire in cylinder 4. The ECM watches the crankshaft spin thousands of times per minute. When cylinder 4 fires correctly, the crankshaft speeds up slightly. When it doesn’t, the ECM catches that dip and starts counting misfire events.

Once those events hit a specific threshold, the Malfunction Indicator Lamp (MIL) lights up and stores the P0304 code.

Two types of misfires matter here:

• Type A misfire — severe enough to damage your catalytic converter. The MIL flashes rapidly. Pull over soon.
• Type B misfire — less severe, hurts fuel economy and emissions. The MIL stays steady.

A flashing MIL is urgent. Unburned fuel entering the exhaust can melt the catalytic converter’s ceramic substrate, turning a $120 coil replacement into a $1,500+ repair bill.

Where Is Cylinder 4 on Your Toyota?

This depends entirely on which engine you’ve got. Don’t guess — check the table below.

For inline-four engines like the 2.5L 2AR-FE, cylinder 4 sits at the far end of the engine block — away from the timing chain. On the transverse 3.5L 2GR-FE V6, it’s the middle cylinder on the front bank facing the radiator, which is actually the easiest one to reach.

The 3 Root Causes of Toyota P0304

Every cylinder 4 misfire traces back to one of three problems: bad spark, bad fuel, or a mechanical failure. Start with spark — it’s responsible for roughly 75% of all Toyota misfire codes.

Ignition Problems (Start Here First)

Toyota uses a Coil-on-Plug (COP) system, meaning each cylinder gets its own dedicated ignition coil. These coils transform 12 volts into tens of thousands of volts to fire the spark plug.

Common ignition failures on Toyota engines:

• Cracked coil boot — high voltage arcs to the cylinder head instead of the spark plug
• Failed coil winding — thermal degradation breaks down the internal insulation
• Worn spark plugs — a wide electrode gap demands more voltage than the coil can deliver
• Oil-contaminated spark plug — leaking valve cover gaskets coat plugs with oil, killing the spark

Toyota specifies iridium-tipped plugs from Denso or NGK. These have a finer center electrode that fires more reliably at lower voltages. But even iridium plugs reach their limit around 100,000–120,000 miles. If yours are overdue, that’s your first suspect.

Fuel Delivery Problems

A lean misfire happens when cylinder 4 doesn’t get enough fuel to ignite. Toyota’s sequential fuel injection system delivers a precise fuel pulse to each cylinder independently — so one bad injector can cause a single-cylinder misfire without affecting the others.

Signs you’ve got a fuel problem:

• Long-Term Fuel Trim (LTFT) reading above +10% on the affected bank
• Misfire gets worse under load, not at idle
• P0304 combined with a P0171 (system lean) code

Direct injection engines like the newer Camry and Tacoma are more prone to injector clogging because the injector sits directly inside the combustion chamber, exposed to intense heat and carbon buildup.

Mechanical Problems (Low Compression)

When spark and fuel check out fine, the problem lives inside the engine itself. Cylinder 4 needs a tight seal to compress the air-fuel mixture enough to ignite it. Common mechanical failures include:

• Burnt or stuck valve — carbon buildup prevents the valve from sealing
• Worn piston rings — pressure escapes into the crankcase
• Blown head gasket — combustion gases leak between cylinders or into the coolant

A healthy Toyota engine produces 160–200 PSI of compression. If cylinder 4 reads more than 15% lower than the others, you’ve found your problem.

Toyota-Specific Issues That Cause P0304

Some Toyotas have known patterns. Check if your model matches before you start replacing parts randomly.

2010–2015 Prius: The Cold Soak Problem

This is the big one. The 1.8L 2ZR-FXE in the third-gen Prius has a well-documented head gasket issue near cylinder 4. Here’s what happens:

When the engine sits overnight, residual cooling system pressure pushes a tiny amount of coolant through a microscopic gasket breach into cylinder 4. At startup, that coolant fouls the spark plug and triggers a violent misfire — often with a loud clacking noise from the hybrid transaxle. Once the coolant burns off, the misfire disappears until the next cold start.

If your Prius P0304 only happens on cold mornings below 40°F, suspect the head gasket first.

Toyota also released T-SB-0010-12 for 2010–2013 Prius models, which addresses cold-start misfires through PCM reprogramming.

2010–2015 Prius: EGR Imbalance

The EGR system on the 2ZR-FXE recirculates exhaust gases through individual ports for each cylinder. On high-mileage engines, carbon paste builds up near the cylinder 4 intake runner. When cylinders 1–3 ports clog first, cylinder 4 gets hit with a disproportionate amount of inert exhaust gas, causing a misfire that’s worst at highway cruising speeds.

Toyota’s fix: T-SB-0027-16 recommends replacing the EGR valve and intake manifold with a redesigned version that improves flow distribution.

2010–2012 Camry (2AR-FE): VVT Gear Issues

If your Camry throws a P0304 with a cold-start rattle, T-SB-0041-13 applies to you. The internal locking pin inside the Variable Valve Timing (VVT) gear can fail, causing the gear to oscillate and throw off valve timing during startup. The fix involves replacing the intake camshaft timing gear — a moderately complex job that requires timing chain access.

How to Diagnose Toyota P0304 Step by Step

Don’t throw parts at it. Follow this sequence and you’ll find the problem without wasting money.

Step 1: Read the Freeze Frame Data

Plug in an OBD-II scanner and retrieve the P0304 code plus freeze frame data. Note the engine RPM, coolant temperature, and engine load when the misfire occurred. A flashing MIL means the misfire is active right now — don’t drive it.

Step 2: The Swap Method (Most Efficient First Move)

This is the fastest diagnostic technique for ignition and fuel issues, and it costs nothing.

1. Swap the cylinder 4 coil to cylinder 1. Clear codes and drive. If the code changes to P0301, the coil is bad.
2. If P0304 stays, swap the cylinder 4 spark plug to cylinder 2. If it becomes P0302, the plug is bad.
3. If P0304 stays, swap the cylinder 4 injector to cylinder 3. A shift to P0303 confirms a faulty injector.

If the misfire follows the part to a new cylinder, you’ve found your culprit without any guesswork.

Step 3: Run a Compression Test

If swapping all three components doesn’t move the code, grab a compression gauge. Crank each cylinder and record the readings. Anything more than 15% below the average points to a mechanical problem in cylinder 4.

Follow a low compression reading with a leak-down test to pinpoint exactly where pressure is escaping:

• Air from the intake pipe → leaking intake valve
• Air from the tailpipe → leaking exhaust valve
• Air from the dipstick tube → worn piston rings
• Bubbles in the coolant reservoir → blown head gasket

OEM vs. Aftermarket Parts: Does It Matter?

Yes, it absolutely matters for Toyotas. The ECM’s timing and ignition logic calibrates specifically for OEM component specs.

Budget aftermarket coils often skip the internal protection circuit that Denso builds into units like the Denso 673-1309. Without it, electromagnetic interference can corrupt crankshaft position sensor signals and cause erratic misfires that move between cylinders — or even fry the PCM’s ignition driver.

What Does Fixing Toyota P0304 Cost?

Here’s the honest breakdown — from the cheap fix to the expensive one.

The spark plug and coil swap are the logical starting points — they’re cheap, quick, and solve the problem roughly 75% of the time. Head gasket repairs are a last resort, but ignoring a confirmed misfire long enough can turn a $120 coil job into one.

Preventing Toyota P0304 Before It Happens

The P0304 code is almost always preventable with basic maintenance. Here’s the schedule worth following:

Stick to these intervals, use Denso or NGK iridium plugs, and your cylinder 4 will keep firing cleanly for the long haul. After completing any repair, clear the DTC and run a full OBD-II drive cycle to confirm the misfire monitor shows “Complete” — that’s your confirmation the fix actually worked.