P0035 Code on a 2017 Ford Fiesta 1.6L EcoBoost – No Check Engine Light? Here’s How I Diagnosed It
Introduction
Every now and then, you run into a vehicle that throws a curveball—like setting a fault code without turning on the check engine light. That’s exactly what I had with a 2017 Ford Fiesta equipped with the 1.6L EcoBoost engine.
At first glance, it didn’t seem like much. No warning lights, no complaints from the driver. But as any experienced tech knows, stored codes can point to underlying issues that haven’t fully surfaced yet.
Here’s how I approached diagnosing a P0035 turbo bypass valve circuit high code, step by step, and how you can tackle something similar.
Vehicle Concern
Vehicle: 2017 Ford Fiesta 1.6L EcoBoost
Customer Complaint: Codes stored in system
Driveability Issues: None reported
Check Engine Light: OFF
This is already an interesting starting point. A stored code with no active symptoms often means:
Intermittent issue
Historical fault
Condition only occurs under specific operating conditions
Initial Inspection & Symptoms
I always start simple.
No warning lights on the dash
Engine ran smoothly at idle
No noticeable lack of power during a quick check
No abnormal noises from the turbo
At this point, nothing mechanical was jumping out. That’s a good sign—it usually means we’re dealing with something electrical or intermittent rather than a hard failure.
Diagnostic Trouble Code
The scan tool revealed:
P0035 – Turbocharger/Supercharger Bypass Valve “A” Control Circuit High
This code is tied to the turbo bypass valve (also called a diverter valve).
What This Code Means
“Circuit High” doesn’t mean high boost—it means the PCM is seeing higher-than-expected voltage in the control circuit.
In real-world terms, that usually points to:
Open circuit (broken wire or unplugged connector)
High resistance (corrosion, poor pin fit)
Failed solenoid (internal open)
Diagnostic Process
Here’s exactly how I approached it and why.
Step 1: Verify Code Status
Before touching anything, I checked whether the code was:
Current
Pending
History
This matters more than most techs realize.
👉 In this case, there was no check engine light, which strongly suggested the code was history or intermittent.
Why this matters:
You don’t want to chase a ghost. If it’s not currently failing, your job becomes reproducing the condition—not just replacing parts.
Step 2: Clear Code and Plan a Road Test
I cleared the code and planned a road test under boost conditions.
That’s key.
This system only operates when:
The turbo is building pressure
The PCM is actively controlling the bypass valve
Driving gently won’t trigger it—you need load.
Step 3: Visual Inspection (Critical on Turbo Cars)
Before even grabbing a meter, I inspected the bypass valve and wiring.
On EcoBoost engines, heat is a big factor.
I checked:
Connector condition
Harness routing near the turbo
Signs of melting, brittleness, or oil contamination
Pin tension inside the connector
Why this matters:
Ford turbo engines are known for:
Heat-related wiring damage
Oil intrusion into connectors
Loose or spread terminals
This step alone solves a lot of these cases.
Step 4: Understand the Circuit
Before testing, you need to know what you’re testing.
Typical setup:
One wire = 12V power (VPWR)
One wire = PCM-controlled ground (low-side driver)
If the PCM sees voltage where it shouldn’t, it flags “circuit high.”
Step 5: Voltage Testing
With the connector unplugged, I checked:
Key On Engine Off (KOEO):
Verified 12V supply at the power wire
Checked control wire voltage
What I’m looking for:
If the control wire is stuck high → possible open circuit
If no voltage on power side → supply issue
Step 6: Actuator Test with Scan Tool
Using a bidirectional scan tool, I commanded the bypass valve on and off.
I listened and watched for:
Audible click
Any change in engine behavior
Current draw (if available)
No response = big clue.
Step 7: Resistance Test of the Valve
Next, I unplugged the valve and checked resistance across the terminals.
Typical spec:
Roughly 20–40 ohms (varies slightly)
Results interpretation:
OL (open loop) → bad solenoid
Extremely high resistance → failing internally
This is a quick and reliable way to confirm a faulty valve.
Key Findings
No active symptoms
No MIL illuminated
Code indicated an electrical fault, not mechanical
System likely functioning most of the time
This strongly pointed toward:
👉 An intermittent electrical issue or a failing solenoid inside the bypass valve
Final Diagnosis / Recommendation
Based on testing and known failure patterns, the most likely cause is:
Faulty turbo bypass valve solenoid (internal open or intermittent failure)
Recommended Action:
Inspect and repair connector if needed
Replace bypass valve if resistance is out of spec or no actuation occurs
Clear codes and perform boost-loaded road test
Common Causes of This Issue
This isn’t just a Fiesta problem—you’ll see this across many turbocharged vehicles.
Typical causes include:
Failed bypass/diverter valve solenoid
Heat-damaged wiring near turbocharger
Oil contamination in electrical connectors
Broken or chafed harness
Poor terminal contact (spread pins)
Rarely: PCM driver failure
Tips for DIYers or Buyers
If you’re tackling this yourself or inspecting a used turbo vehicle:
🔧 What to Check
Look closely at wiring near hot components
Don’t assume the turbo itself is bad
Always test the circuit before replacing parts
⚠️ Important Tip
A stored code with no symptoms doesn’t mean “ignore it.”
It often means:
The problem is just starting
You caught it early
That’s actually the best time to fix it—before it becomes a drivability issue.
Tools Used
Professional scan tool (Autel or equivalent)
Digital multimeter
Basic hand tools
When to Refer to a Professional
If you don’t have:
A scan tool with actuator control
Wiring diagrams
Electrical diagnostic experience
It’s worth getting a professional involved. Electrical issues can get expensive if misdiagnosed.
Conclusion
This case is a perfect example of why diagnostics matter more than parts replacement.
A P0035 code might sound serious, but in reality, it’s often:
A simple electrical issue
A failing solenoid
Or a wiring problem caused by heat
By approaching it step-by-step—checking code status, verifying the circuit, and testing the component—you can pinpoint the issue without guesswork.
And that’s what separates a proper diagnosis from just throwing parts at a problem.
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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