2019 Kia Soul Misfire & Soft Brake Pedal Diagnosis: Step-by-Step Professional Breakdown
Introduction
When a vehicle comes in with multiple issues, it’s important to stay methodical and avoid jumping to conclusions. In this case, I was working on a 2019 Kia Soul that had two primary concerns: a check engine light with misfire codes and a soft brake pedal.
Both issues required separate diagnostic paths, but as always, the goal was the same—identify the root cause efficiently and avoid unnecessary parts replacement.
Vehicle Concern
The customer reported two problems:
Check engine light illuminated
Noticeable engine misfire
Soft brake pedal with reduced confidence during braking
Any time you have both drivability and safety concerns, it’s critical to prioritize correctly while still addressing everything thoroughly.
Initial Inspection & Symptoms
I started with a quick visual and operational check:
Engine was running rough, especially at idle
No obvious abnormal engine noises
Brake pedal felt soft and spongy, not firm like it should be
Before diving deeper, I always verify the basics:
Fluid levels
Obvious leaks
Anything visually out of place
Nothing obvious stood out initially, so it was time to hook up the scan tool.
Diagnostic Trouble Codes
Using a scan tool, I pulled the following codes:
P0116 – Engine Coolant Temperature Circuit Range/Performance
P0300 – Random/Multiple Cylinder Misfire
P0302 – Cylinder 2 Misfire (Pending)
P0304 – Cylinder 4 Misfire (History)
This gave me a clear direction—focus on ignition and possible contributing factors like sensor input.
Diagnostic Process
Step 1: Verify Mechanical Integrity
Before chasing ignition or fuel issues, I always confirm the engine is mechanically sound.
I performed a compression test across all cylinders.
All cylinders measured approximately 150 PSI
That’s exactly what I want to see—consistent and healthy compression across the board. This ruled out:
Burnt valves
Worn piston rings
Major internal engine issues
Now I knew I could confidently move on to ignition and fuel.
Step 2: Inspect Spark Plugs
Next, I removed the spark plugs for inspection.
What I found was significant:
Plugs were heavily worn
One plug had the electrode worn down more than halfway
This is a classic cause of misfires. As spark plugs wear, the ignition system has to work harder to fire the mixture, eventually leading to inconsistent combustion.
At this point, the plugs alone could explain the misfire—but I never stop there.
Step 3: Analyze Misfire Pattern
The codes showed:
Active misfire on Cylinder 2
Previous misfire on Cylinder 4
This pattern often points toward ignition coil issues, especially on Hyundai/Kia platforms where coil failures are common.
Instead of guessing, I evaluated the likelihood:
Worn plugs = confirmed issue
Coil failure = highly probable, especially on cylinder 2
Given the condition of the plugs and the active misfire, replacing components was justified.
Step 4: Address Coolant Temperature Code (P0116)
The P0116 code indicates an issue with the engine coolant temperature signal.
While not directly causing a single-cylinder misfire, it can contribute to:
Improper fuel mixture
Poor cold start performance
Overall drivability issues
This is something to monitor after ignition repairs. If the code returns, further testing of the ECT sensor and thermostat operation would be required.
Step 5: Brake System Diagnosis
Now shifting focus to the soft brake pedal.
This is a safety concern, so I approached it carefully.
Visual Inspection:
Lifted the vehicle
Inspected all four corners
Checked:
Brake calipers
Brake hoses
Steel lines
Master cylinder area
Result:
No external leaks found anywhere in the system
That’s a key turning point in brake diagnostics.
Step 6: Evaluate Pedal Feel
With no external leaks, I evaluated pedal behavior:
Pedal felt soft and lacked firmness
Condition remained consistent
This strongly suggests internal hydraulic bypass, most commonly inside the master cylinder.
Key Findings
Engine compression is good and consistent
Spark plugs are severely worn
Misfire primarily affecting cylinder 2
High likelihood of ignition coil failure on cylinder 2
No external brake fluid leaks present
Brake pedal condition indicates internal master cylinder failure
Final Diagnosis / Recommendation
For this 2019 Kia Soul, the following repairs are recommended:
Engine:
Replace all spark plugs
Replace ignition coil on cylinder #2
Clear codes and re-evaluate system operation
Brake System:
Replace the brake master cylinder
Perform proper brake bleed procedure after installation
These repairs address both the drivability issue and the braking safety concern.
Common Causes of This Issue
Misfires on Kia/Hyundai Engines:
Worn spark plugs
Failing ignition coils
Carbon buildup (especially on GDI engines)
Faulty fuel injectors
Sensor input issues (like ECT)
Soft Brake Pedal:
Internal master cylinder failure
Air in brake system
Brake fluid contamination
Failing ABS hydraulic unit (less common)
Tips for DIYers or Buyers
If you’re diagnosing a similar issue, keep these tips in mind:
Always check compression first before replacing parts
Don’t overlook spark plug condition—it’s often the root cause
Misfires that move between cylinders usually point to ignition components
A soft pedal with no visible leaks almost always indicates a master cylinder issue
Safety Note:
Brake system work is critical. If you're not confident performing hydraulic repairs and bleeding procedures, this is one area where it's best to involve a professional.
Conclusion
This 2019 Kia Soul is a great example of why a structured diagnostic approach matters. Instead of guessing, we confirmed engine health, identified worn ignition components, and properly diagnosed a brake system issue without unnecessary parts replacement.
By separating the concerns and working step-by-step, we were able to confidently pinpoint both problems and recommend the correct repairs.
Call to Action
If you're buying a used vehicle, having a solid inspection strategy can save you thousands. Check out my Used Car Inspection Guide to help you make a smart purchase.
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