P0078 means exhaust valve control solenoid circuit bank 1. The ECM saw a value or response outside its calibrated window for the fuel & air metering system. Real-world fixes usually start with the highest-probability cause below for your year, make, and model - not a parts-cannon guess.
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The primary control device for this circuit is out of range or stuck.
#2Wiring open, short, or corroded connector$50-$250
0%
Harness damage or a bad connector drops voltage or signal integrity.
#3Vacuum leak or intake restriction$40-$200
0%
Unmetered air or blocked flow skews fuel trim and related monitors.
#4ECM software or adaptive limit$0-$150
0%
Adaptives are maxed or a PCM update is needed after a repair.
Symptoms Radar
Check engine light on (or flashing if misfire-related)
Possible rough running or fuel economy drop
Failed emissions readiness or inspection risk
Intermittent stumble under load
Related codes stored with freeze-frame data
Repair Cost
DIY
$40-$280
Shop
$150-$750
Diagnose It
1Confirm P0078 is current (not history-only) and save freeze-frame RPM, load, and fuel trim.
2Inspect connectors and harness for the listed sensor/actuator before replacing parts.
3Command the actuator or graph the sensor live with a scan tool; compare to known-good values.
4Fix the root cause, clear codes, and verify monitors complete on a mixed drive cycle.
FAQ
Can I drive with a P0078 code?
Usually drivable short-term, but fix soon to avoid catalyst damage or a roadside no-start.
How much does it cost to fix P0078?
DIY parts typically land around $40-$280. A shop diagnosis plus repair commonly runs $150-$750, depending on whether it is a sensor, harness, or major component.
What is the most common cause of P0078?
Failed sensor or actuator is the most common starting point (~38%), followed by wiring open, short, or corroded connector.