Toyota 4Runner P0300: Random or Multiple Cylinder Misfire Detected

Toyota 4Runner P0300 Random Misfire Diagnosis

Quick Answer: What is P0300?

On a Toyota 4Runner, the P0300 code triggers when the engine computer detects that multiple cylinders are not firing correctly. For the 4.0L V6 (1GR-FE), this is typically caused by failing ignition coils, worn spark plugs, or carbon buildup on the throttle body.

Emergency Warning: If your Check Engine Light is flashing, pull over immediately. A blinking light means your engine is actively misfiring so severely that raw gasoline is entering the exhaust, which will destroy your expensive catalytic converters in a matter of minutes.

Common Symptoms

Rough Idle: The steering wheel and gear shifter vibrate at stoplights.
Stumbling: The engine feels like it is "missing" a beat during acceleration.
Flashing CEL: Blinking light under heavy load or while off-roading.
Reduced Performance: The 4Runner feels weak when climbing hills or towing.

Most Likely Causes

Failed Ignition Coils (45%): Denso coils are excellent but eventually fail after 120k+ miles of engine heat.
Worn Spark Plugs (30%): If you have over 100,000 miles on your Iridium plugs, the gap is likely too wide.
Dirty Throttle Body: Carbon buildup restricting airflow at idle (very common on 4Runners).
Vacuum Leak: A cracked rubber intake boot or disconnected vacuum hose.

Detailed Repair Cost Breakdown

Estimates for Toyota 4Runner (4.0L V6) in the US market:

Component / Task	Aftermarket Part	OEM Denso/Toyota	Labor Time
Ignition Coil (Each)	$55 - $80	$115 - $155	0.5 hr (DIY Friendly)
Spark Plugs (Set of 6)	$50 - $80	$110 - $160	1.5 - 2.5 hrs

How to Fix P0300 on a Toyota 4Runner

1. The "Idle" Fix: Clean the Throttle Body

If your 4Runner only misfires at idle but runs fine at speed, your throttle body is likely the culprit. Carbon buildup around the butterfly valve prevents the engine from getting enough air at a stop. Cleaning it with a rag and some Throttle Body Cleaner is the most common "cheap fix" for a 4Runner P0300 code.

2. Only Use Denso or NGK Plugs

Toyota engines are extremely sensitive to spark plug resistance. Do not use generic or "budget" brands. Only use Genuine Denso or NGK Iridium plugs. Aftermarket plugs often cause electrical "noise" that the 4Runner's computer interprets as a misfire.

3. The Coil Swap Diagnostic

If your scan tool shows a specific cylinder (like P0304), swap that ignition coil with cylinder 2. If the code moves to P0302, the coil is dead. If you have over 150,000 miles, it is highly recommended to replace all 6 coils at once to prevent future trail breakdowns.