P0078 Code Explained: Exhaust Valve Control Solenoid Circuit High Bank 1 Causes, Symptoms & Fixes

P0078 Code Explained: Exhaust Valve Control Solenoid Circuit High Bank 1 Causes, Symptoms & Fixes

If your OBD-II scanner displays P0078, your vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a higher-than-expected electrical signal in the Bank 1 exhaust valve control solenoid circuit.

The generic OBD-II definition for P0078 is:

Exhaust Valve Control Solenoid Circuit High Bank 1

Depending on the manufacturer, this code may also appear as:

  • Exhaust Valve Timing Control Solenoid Circuit High Bank 1
  • Exhaust Camshaft Oil Control Valve Circuit High Bank 1
  • Exhaust VVT Solenoid Circuit High Bank 1
  • Exhaust Camshaft Actuator Solenoid Circuit High
  • Variable Valve Timing Exhaust Solenoid Circuit High
  • Exhaust Oil Control Valve Open Circuit Bank 1

The exhaust valve control solenoid is responsible for controlling oil flow to the Bank 1 exhaust camshaft actuator (cam phaser). By adjusting exhaust valve timing, the ECM can improve engine performance, fuel economy, idle quality, emissions, and turbocharger efficiency.

Unlike timing-performance codes, P0078 is an electrical fault code. It indicates the ECM sees a circuit voltage or feedback signal that remains higher than expected, often because of an open circuit, disconnected solenoid, broken wiring, or a short to battery voltage.

Common causes include:

  • Failed exhaust VVT solenoid
  • Open solenoid coil
  • Disconnected electrical connector
  • Broken wiring
  • Short to battery voltage
  • Corroded terminals
  • Failed relay or fuse
  • PCM driver failure

Typical symptoms include:

  • Check Engine Light
  • Rough idle
  • Reduced power
  • Poor acceleration
  • Hesitation
  • Increased fuel consumption
  • Hard starting
  • Failed emissions inspection
  • Little or no noticeable symptoms

For additional diagnostic guides, see:

P0078 Quick Facts

ItemInformation
OBD-II CodeP0078
DescriptionExhaust Valve Control Solenoid Circuit High Bank 1
CategoryPowertrain
SystemVariable Valve Timing (VVT)
SeverityModerate
Safe to Drive?Usually for short distances
Typical Repair Cost$100–$900

What Does P0078 Mean?

P0078 means the ECM detects the Bank 1 exhaust valve control solenoid circuit remains at a voltage higher than its expected operating range.

This commonly happens because:

  • The solenoid has an open coil
  • The connector is unplugged
  • The control wire is broken
  • The control circuit is shorted to battery voltage
  • The PCM cannot control the driver circuit

Unlike P0079 (performance/intermittent faults), P0078 specifically indicates an electrical high-voltage condition.

What Is the Exhaust Valve Control Solenoid?

The exhaust valve control solenoid is an electronically controlled oil valve that regulates oil pressure to the exhaust camshaft actuator.

Manufacturers may call it:

  • Exhaust Oil Control Valve
  • Exhaust VVT Solenoid
  • Exhaust Camshaft Actuator Solenoid
  • Oil Control Valve (OCV)
  • Variable Cam Timing Solenoid
  • VCT Solenoid

The ECM constantly adjusts exhaust valve timing based on:

  • RPM
  • Engine load
  • Coolant temperature
  • Oil temperature
  • Throttle position
  • Engine speed

This improves:

  • Idle stability
  • Fuel economy
  • Low-end torque
  • High-RPM horsepower
  • Turbo spool
  • Emissions

How the Circuit Works

Most exhaust VVT solenoids use:

  • One constant battery voltage
  • One PCM-controlled ground

The PCM rapidly switches the ground circuit using pulse-width modulation (PWM).

The ECM monitors:

  • Circuit voltage
  • Current draw
  • Coil resistance
  • Driver feedback
  • Commanded duty cycle

If the ECM commands the circuit low but continues seeing battery voltage, it stores P0078.

What Does “Circuit High” Mean?

“Circuit High” refers to the electrical signal, not engine oil pressure.

It typically means:

  • Open circuit
  • Open solenoid
  • Broken wire
  • Connector unplugged
  • Short to battery voltage
  • Failed PCM driver

It does NOT mean:

  • High oil pressure
  • Advanced timing
  • High engine temperature
  • Excessive valve lift

Symptoms of P0078

Drivers may notice:

  • Check Engine Light
  • Rough idle
  • Poor acceleration
  • Sluggish engine
  • Reduced power
  • Hesitation
  • Increased fuel consumption
  • Failed emissions test
  • Hard starting
  • Engine stalls
  • No obvious symptoms

Many vehicles simply disable variable valve timing until repairs are completed.

Common Causes of P0078

1. Failed Exhaust VVT Solenoid

The internal winding can fail, producing:

  • Open circuit
  • Excessive resistance
  • Intermittent operation

2. Open Solenoid Coil

An open coil prevents current flow entirely.

The ECM sees battery voltage remaining on the circuit.

3. Disconnected Connector

One of the most common causes after engine work.

Always verify the connector is fully locked.

4. Broken Wiring

Damaged wiring near the cylinder head commonly causes:

  • Open circuits
  • Intermittent faults
  • High circuit voltage

Inspect for:

  • Rodent damage
  • Chafing
  • Melted insulation
  • Previous repairs

5. Short to Battery Voltage

The control wire may contact:

  • Injector wiring
  • Ignition wiring
  • Charging system wiring
  • Other power circuits

This prevents the PCM from properly controlling the solenoid.

6. Corroded Connector

Corrosion increases resistance and interrupts normal current flow.

Inspect for:

  • Green corrosion
  • Water intrusion
  • Oil contamination
  • Loose terminals

7. Failed Fuse or Relay

If the power supply is abnormal, the ECM may detect incorrect voltage behavior.

Inspect shared VVT fuses and relays.

8. Incorrect Replacement Solenoid

Wrong aftermarket parts sometimes have:

  • Incorrect resistance
  • Wrong connector
  • Incorrect valve design

Always verify OEM specifications.

9. PCM Driver Failure

Rare, but possible after:

  • Wiring shorts
  • Solenoid failures
  • Water intrusion

Always eliminate external wiring faults first.

Is P0078 Serious?

P0078 is considered moderately serious.

The vehicle usually remains drivable, but:

  • Fuel economy suffers
  • Power decreases
  • Emissions increase
  • Cam timing becomes fixed

Driving should be avoided if:

  • Oil pressure warning appears
  • Engine stalls
  • Severe timing noise develops
  • Multiple cam timing codes appear

Can You Drive With P0078?

Usually yes—for a short distance.

Do not continue driving if:

  • Engine loses significant power
  • Oil pressure drops
  • Timing chain noise develops
  • Vehicle stalls repeatedly

How to Diagnose P0078

Step 1

Scan all stored codes.

Look for related codes including:

  • P0010
  • P0011
  • P0012
  • P0020
  • P0021
  • P0022
  • P0079
  • P0080

Step 2

Verify oil level and oil condition.

Dirty oil won’t usually create P0078 directly, but it can contribute to VVT problems.

Step 3

Locate the Bank 1 exhaust VVT solenoid.

Inspect:

  • Connector
  • Harness
  • Pins
  • Lock tabs

Step 4

Measure coil resistance.

Compare with factory specifications.

Infinite resistance usually indicates an open coil.

Step 5

Verify power supply.

Confirm:

  • Battery voltage
  • Fuse integrity
  • Relay operation

Step 6

Check the control wire.

Inspect for:

  • Opens
  • Shorts
  • Short to battery voltage
  • Damaged insulation

Step 7

Command the solenoid using a bidirectional scan tool.

Watch:

  • Duty cycle
  • Current draw
  • Camshaft angle
  • Live data

Step 8

Repair the fault.

Clear codes.

Complete a drive cycle.

Verify P0078 does not return.

Common Repairs

RepairTypical Cost
Reconnect connector$0–50
Repair wiring$150–600
Replace VVT solenoid$150–500
Replace connector$100–250
Oil service$60–180
Replace PCM$1,000–2,500

Common Diagnostic Mistakes

Avoid these errors:

  • Replacing the cam phaser first
  • Ignoring an unplugged connector
  • Skipping wiring inspection
  • Assuming dirty oil caused the electrical fault
  • Forgetting to inspect Bank 1
  • Ignoring blown fuses
  • Replacing the PCM before testing the circuit

Sometimes the “major engine problem” turns out to be a connector that was left unplugged during yesterday’s valve cover gasket replacement. Humbling? Yes. Expensive? Only if you skip the basics.

Frequently Asked Questions

What does P0078 mean?

The ECM detects a high electrical signal in the Bank 1 exhaust valve control solenoid circuit.

Can a bad VVT solenoid cause P0078?

Yes.

An open internal coil is one of the most common causes.

Can dirty oil cause P0078?

Not directly.

Dirty oil causes hydraulic VVT problems, while P0078 is primarily an electrical circuit fault.

Can I drive with P0078?

Usually for a short distance, provided there are no oil pressure warnings or severe drivability issues.

Is P0078 expensive to fix?

Most repairs range from $150–500.

Major wiring repairs or PCM replacement increase costs significantly.

Final Thoughts

P0078 indicates that the Bank 1 exhaust valve control solenoid circuit remains electrically high, preventing the ECM from properly controlling exhaust camshaft timing.

The most common repairs involve:

  • Replacing a failed exhaust VVT solenoid
  • Repairing broken wiring
  • Fixing damaged connectors
  • Correcting a short-to-voltage condition
  • Replacing a damaged PCM driver (rare)

Begin with the simple checks first. Variable valve timing may sound like advanced engineering—and it is—but more than a few P0078 repairs have ended with the timeless automotive diagnosis of, “Yep… that connector was never plugged back in.”

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Continue your diagnosis with these Pro Street guides:

You can also browse the complete OBD-II Diagnostic Code Library, plus the B-Code and C-Code libraries on Pro Street Online.