Unexpected Injector Failure in Toyota Vehicles!? Mysterious Engine Malfunction and Correct Diagnostic Procedures – [Case Study Explanation] 1NZ-FE Engine P0304 Error Code – Investigating the True Cause of No.4 Cylinder Misfire

At the beginning of 2025, I received a call (consultation) from a fellow professional.

They had sold a 10-year-old vehicle with less than 70,000 kilometers, assuming it would be reliable since Toyota has an image of never breaking down. However, six months after delivery, the engine warning light came on, along with noticeable engine problems (especially vibration during idling).

The symptoms of a specific cylinder not functioning were apparently intermittent with poor reproducibility, but they confirmed the error code P0304, indicating a misfire detection in the No.4 cylinder.

They installed aftermarket ignition coils, aftermarket spark plugs, and a rebuilt air flow sensor (does such a thing even exist?!) based on their best guess, but the symptoms reappeared after a while with no improvement.

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2014 (Heisei 26) DBA-NCP141 1NZ-FE CVT Mileage 67,000km

For this model year’s 1NZ-FE, the engine control self-diagnosis system identifies the misfiring cylinder.

The system works by having a signal plate with multiple teeth attached to the crankshaft rotate, and as the teeth pass by a magnetic sensor (crank angle sensor) fixed to the cylinder block, rotation signals are obtained. This signal waveform has characteristics specific to certain cylinder misfires, which is how they are identified.

Therefore, when the error code indicates a No.4 cylinder misfire, it’s reasonable to suspect an issue with the No.4 cylinder.

I don’t deny that ignition system parts like ignition coils and spark plugs often cause problems.

However, replacing all ignition system parts from the beginning is not a reasonable approach.

The proper first step would be to rearrange the ignition coil and spark plug that were installed in the No.4 cylinder to different locations (for example, swap the ignition coil with the No.2 cylinder and the spark plug with the No.3 cylinder), and then observe changes in error codes (transition of misfiring cylinder numbers) when the problem occurs again.

I understand the desire for a quick solution when problems appear six months after purchasing a used car, but when you replace all ignition coils and spark plugs for all cylinders as in this case, you enter a vicious cycle of confusion, impatience, and worry, leading to replacement of parts that are unlikely to be the cause.

Fortunately, the symptoms appeared when I received the vehicle.

Initial defects in aftermarket parts are rare, but let’s get back to the standard (genuine) parts.

The trustworthy and reliable red box. There are spark plug part numbers that only exist as genuine parts. We avoid using generic parts whenever possible.

The symptoms continued even after replacing with genuine ignition parts. Now we can finally start the proper diagnosis.

Previously, I had a 1NZ with an EGR valve issue outside of the ignition system, so I checked it just in case, but it was normal. Let’s examine the No.4 cylinder issue based on the fundamental principles of internal combustion engines (ignition, fuel, compression). We’ll assume the ignition is fine since we’re using new genuine parts.

Next, fuel. With the engine running, I listened to the injector operation sounds with a stethoscope. I couldn’t hear the No.4 injector working. When measuring the resistance value:

As you can see, it significantly deviates from the reference value. Injector failures, which are relatively common in imported cars, are rarely experienced in Japanese cars (especially Toyota).

This is a valuable sample that we’ve obtained.