BMW M235i P0300 Misfire – The Fault Beneath the Surface

“It’s had new coils, spark plugs, and injectors fitted, but it still misfires.”

That’s how Mrs C described the ongoing issue with her BMW M235i when she visited Roade Main Garage, Northampton. The problem was inconsistent; the engine light would flash, performance would dip, then everything would return to normal.

Under normal driving conditions, it seemed fine, but during cold starts or heavy acceleration, it ran unevenly and lacked its usual responsiveness.

This left Mrs C frustrated, especially as she’d already visited other garages for help, but the fault continued.

That’s when Mrs C brought her M235i to our BMW experts at Roade Main Garage, Northampton.

Instead of relying on assumptions or more part replacements, our team followed a systematic, evidence-based repair process to reveal the underlying cause and solve the BMW intermittent misfire once and for all.

Listening First: The Initial Inspection

At Roade Main Garage, Northampton, every repair begins with one step… Listening.

Our team began the process by discussing the problem with Mrs C, taking the time to understand exactly when the misfire occurred.

She explained that her BMW M235i misfired when cold-starting and under acceleration, flashing the engine warning light before clearing once the engine reached its operating temperature.

She also mentioned that spark plugs, coils, and injectors had already been replaced elsewhere, yet annoyingly, the fault persisted.

Knowing what had already been tried and when the problem appeared helped us plan our next steps efficiently.

Our BMW experts conducted a visual inspection, checking for damaged wiring, loose connectors, and contamination. Everything looked good, so the next stage was to verify the fault during a test drive.

Recreating the same driving conditions Mrs C described, we observed a brief hesitation under acceleration, which disappeared once the car was warm.

Once we got back to the garage, we connected to our dealer-level scan tool to identify stored fault codes (if any).

We discovered the following fault codes were present:

P0300 – Random or multiple cylinder misfire detected.
P0304 – Cylinder 4 misfire detected.
P0306 – Cylinder 6 misfire detected.

These BMW fault codes confirmed the misfire was present, but didn’t identify the root cause. To get to the bottom of it, a deeper inspection was required.

Finding the True Cause of the BMW M235i P0300 Misfire

After confirming the BMW M235i P0300 misfire, our BMW experts at Roade Main Garage, Northampton, began tracing the cause of the misfire.

Because the hesitation occurred mainly from a cold start, our technicians examined the ignition system and the parts that had already been replaced. As Mrs C had mentioned that spark plugs, coils, and injectors had recently been replaced elsewhere, it seemed the logical place to start.

Upon a closer inspection, the replacement parts looked new and correctly installed. Yet, something immediately stood out to us… The spark plugs. They were non-genuine parts made by BMW’s original supplier, meaning they were manufactured to slightly different tolerances.

Even slight design variations, like this, can affect combustion accuracy in high-performance engines, such as the BMW M235i, where precise ignition timing is crucial.

To confirm their findings, our team carried out a BMW misfire diagnosis using an oscilloscope.

Each cylinder’s ignition waveform was analysed step by step for any irregularities:

The primary and secondary ignition waveforms were recorded.
We compared the readings under load and during a cold start.
Checked coil activity, injector timing, and wiring condition.

The oscilloscope results showed inconsistent signals with no discernible pattern, ruling out coil or injector malfunction, leading us to focus on the spark plugs.

Further testing confirmed that the non-genuine spark plugs were responsible for the BMW M235i P0300 misfire. Even though BMW’s supplier makes them, they did not meet the required specification, resulting in unstable ignition during cold operation.

We’ve encountered this issue before, and it’s further discussed in our spark plug blog, which highlights why genuine BMW spark plugs are crucial for reliable ignition performance.

Repairing the BMW M235i P0300 Misfire the First Time

With the BMW misfire diagnosis confirmed, our BMW experts called Mrs C to discuss the findings.

We explained that the spark plugs fitted elsewhere were made by BMW’s supplier but didn’t match BMW’s precise specification. Even slight design differences can affect combustion timing and ignition stability.

Our recommendation was to fit six genuine BMW spark plugs to restore optimal ignition and prevent the misfire problem from recurring.

At Roade Main Garage, Northampton, transparent and honest communication is our top priority. We always ensure you understand what’s being done and why before any repairs begin. No work begins until you say so; you’re in complete control.

Once the repair was authorised, our team followed BMW’s repair procedure step by step. Here are the steps taken by our experts:

Allowed the engine to cool fully before plug removal to avoid thread damage.
Removed the old plugs and inspected the plug wells and coil packs for oil or carbon traces.
Cleaned each plug seat to ensure a precise seal and accurate torque.
Checked every new spark plug for the correct gap and resistance before installation.
Fitted the genuine BMW plugs to BMW’s torque specification using calibrated equipment.
Re-fitted and tested the coils to confirm they were seated securely and operating correctly.

This careful process ensured the ignition system functioned perfectly, eliminating the BMW M235i P0300 misfire and restoring smooth, consistent power to Mrs C’s car.

Proving the BMW M235i Misfire Was Fixed

With the repair complete, our next step was to confirm that the BMW M235i P0300 misfire had been fully resolved. At Roade Main Garage, Northampton, every successful repair ends with a validation stage, ensuring the fault has been correctly diagnosed and fixed.

To confirm the repair, our BMW experts rechecked the ignition system using the oscilloscope. Each cylinder’s ignition waveform was analysed carefully to confirm optimal function.

To verify performance under real-world conditions, the car was started from cold and taken on an extended test drive. Throughout the drive, the BMW M235i delivered smooth, consistent power with no hesitation, misfiring or warning light. The intermittent misfire didn’t reappear.

A final scan revealed that all previous fault codes had cleared, confirming the BMW M235i P0300 fault had been successfully repaired.

This final stage confirmed the issue was resolved and the engine was running perfectly once again, and was ready to be collected by Mrs C.

Successful BMW M235i P0300 Misfire Repair

When Mrs C collected her BMW M235i, our BMW experts at Roade Main Garage, Northampton, took the time to explain each part of the process. The oscilloscope data, test reports, and final scan results provided clear proof that the misfire had been completely repaired.

By applying a systematic and evidence-based diagnostic process, our team efficiently and accurately resolved the BMW M235i P0300 misfire, preventing the unnecessary replacement of working components.

Mrs C left confident that her BMW M235i was performing perfectly once again, with smooth, reliable power restored.

BMW M235i Misfire? Get It Checked at Roade Main Garage

When your BMW 2 Series hesitates, misfires, or shows an engine warning light, it’s a sign that something isn’t right. Addressing it early helps prevent further issues and costly repairs.

At Roade Main Garage, Northampton, our BMW experts use advanced dealership-level equipment and manufacturer technical data to identify and fix problems like the BMW M235i P0300 misfire.

You’ll benefit from dealer-level accuracy with the friendly, straightforward service of an independent garage that values transparency and trust.

Why Drivers Choose Roade Main Garage

BMW owners across Northampton, Milton Keynes and the surrounding areas rely on Roade Main Garage for precision repair and trusted local service. Here’s why: