Intermittent Misfire on a 2021 Mercedes-Benz GLC 300: Diagnosing a Problem That Disappeared
Introduction
One of the most frustrating things in diagnosing car problems is when a vehicle shows clear fault codes—but refuses to act up when you test it. That’s exactly what happened with this 2021 Mercedes-Benz GLC 300.
At first glance, the vehicle came in with multiple stored codes, including a cylinder 3 misfire and fuel mixture imbalance. These are the kinds of symptoms that can point to serious engine issues if ignored. But here’s the twist—the vehicle ran perfectly during testing.
So how do you handle a situation where the evidence says there’s a problem, but the car doesn’t cooperate?
Let’s walk through this real-world automotive troubleshooting case step by step.
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Vehicle Overview
The vehicle in question is a 2021 Mercedes-Benz GLC 300, equipped with the M264 2.0L turbocharged direct injection engine. These engines are efficient and powerful, but like many modern setups, they rely heavily on precise fuel control and electronics.
Vehicle Concern
The vehicle initially came through inspection with a check engine light and multiple stored fault codes. There were no noticeable drivability issues reported at the time, but the presence of cylinder-specific faults raised concern.
In situations like this, especially during a used car inspection, you don’t ignore those warning signs. Even if the car feels fine, stored faults can indicate underlying problems waiting to surface.
Initial Inspection
A full system scan revealed several codes across different modules, including communication and voltage-related faults. More importantly, the engine control module showed cylinder-specific issues.
At this point, the key is not to get distracted by every code. Focus on patterns.
When diagnosing car problems, especially on modern vehicles, you have to separate primary faults from secondary ones caused by voltage drops or communication errors.
Codes
The engine-related codes included:
Cylinder 3 misfire detected
Cylinder 3 mixture imbalance (lean condition)
Injector correction at limit for cylinder 3
This combination is important. It tells you the engine control module had to compensate heavily for one cylinder—and eventually couldn’t keep up.
That’s not random. That’s a clue.
Diagnostic Process
The first step was verifying whether the issue was active or historical.
Live data was monitored at idle using misfire counters. Surprisingly, all cylinders showed zero misfires. The engine ran smooth, with no roughness or hesitation.
At this point, many techs make the mistake of either:
Replacing parts too quickly
Dismissing the issue entirely
Neither approach is correct.
Next step: clear all codes and perform a controlled road test.
The vehicle was driven under various conditions—idle, cruise, and moderate acceleration. These are the scenarios where fuel-related issues typically show up.
During the test drive:
No misfires occurred
No hesitation was felt
No warning lights returned
A full rescan confirmed zero returning codes.
Key Findings
At this stage, we had a vehicle that:
Previously stored cylinder-specific fuel and misfire faults
Currently runs perfectly
Does not reproduce the issue under testing
This is a classic intermittent condition.
Based on experience, this pattern strongly suggests an early-stage injector issue—specifically, one that only fails under certain conditions and then recovers.
Final Diagnosis
Even though the issue could not be duplicated during testing, the stored data told a clear story.
The decision was made to replace the fuel injector on cylinder 3.
Why?
Because the combination of:
Cylinder-specific lean condition
Injector correction limits reached
Misfire history
…points directly to a fuel delivery issue in that cylinder.
In real-world diagnostics, sometimes you have to trust the data the vehicle recorded, even if it’s not acting up in front of you.
Common Causes
This type of issue is more common than people think, especially on direct injection engines.
Typical causes include:
Failing or restricted fuel injector
Poor injector spray pattern
Intermittent internal injector sticking
Fuel quality issues (less common, but possible)
Direct injection systems operate at very high pressure, and even small inconsistencies can cause noticeable imbalances.
Lessons for Buyers / DIYers
This case is a perfect example of why used car inspection tips matter.
A vehicle can:
Run perfectly
Show no symptoms
Still have underlying issues stored in the system
If you’re only relying on how a car feels during a test drive, you’re missing half the story.
Modern vehicles log everything. You need to look at that data.
Also, don’t assume that no current symptoms means no problem. Intermittent faults are often the most expensive ones if ignored.
And finally—don’t rush to replace parts without understanding the pattern. But when the data consistently points in one direction, don’t ignore it either.
Don’t Get Burned on Your Next Used Car
If you’re thinking about buying a used car—or even if you already have one—this is exactly the kind of problem that can slip through the cracks and cost you big money later.
After 35+ years in the automotive field, I’ve seen firsthand how often people end up with vehicles that have hidden issues just like this.
That’s why I put together a simple Used Car Guide to help you understand what to look for and how to avoid making a costly mistake.
It’s a quick, practical guide designed to give you the right mindset and approach before you buy.
👉 https://stan.store/BobbyWhiteside
AFFILIATE DISCLAIMER
Disclaimer: Some links in this post may be affiliate links. I may earn a small commission at no extra cost to you. I only recommend tools I personally use and trust in real-world diagnostics.
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