P0078🔧 Variable valve timing (exhaust camshaft oil control solenoid)Stop and fix promptly

P0078: Exhaust Valve Control Solenoid Circuit Bank 1 (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 2 cited source(s)

What P0078 means

On engines with variable valve timing, the engine computer (ECM/PCM) moves the camshaft by sending engine oil through a control solenoid. OBD-Codes.com explains that the computer drives this solenoid with a pulse width modulated (PWM) signal and watches that signal; if the voltage is out of specification or intermittent, it sets P0078 and turns on the check engine light. Bank 1 is the side of the engine with cylinder number 1, and OBD-Codes.com says the exhaust valve control solenoid is usually on the exhaust manifold side of the cylinder head. It is a generic code that applies to all makes from 1996 on, though repair steps vary by model. RepairPal describes the same part: a solenoid that uses oil pressure to let the PCM adjust when the exhaust valves open and close, with P0078 set when the PCM finds an issue with the solenoid's signal. The sources differ in a few ways. OBD-Codes.com describes a purely electrical fault (bad connections, a faulty solenoid, or a short to power or ground), while RepairPal lists engine oil problems among the common causes and an oil and filter change as the common repair. RepairPal's severity (Urgent) and its 'engine may not start' symptom are also stronger than anything OBD-Codes.com lists, and RepairPal's title leaves out the bank. OBD-Codes.com says this code is similar to P0079 and P0080, which cover the same bank 1 solenoid circuit reading low and high, and may be accompanied by P0027.

Severity: can I keep driving?

Stop and fix promptly

Get it checked soon and avoid hard driving until then. RepairPal rates P0078 Urgent, meaning immediate action is needed, and lists decreased engine performance, higher fuel use and, in some cases, an engine that may not start. OBD-Codes.com gives no severity rating and lists milder symptoms: the check engine light, poor acceleration and lower fuel economy. Check your engine oil level right away, since RepairPal lists low or wrong-spec oil among the causes.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Poor wiring harness connections or corroded terminals (OBD-Codes.com), or another wiring issue (RepairPal)
  2. Faulty exhaust valve control solenoid (OBD-Codes.com, RepairPal)
  3. Short to power in the solenoid wiring (OBD-Codes.com)
  4. Short to ground in the solenoid wiring (OBD-Codes.com)
  5. Engine oil level too low (RepairPal only)
  6. Engine oil that doesn't meet the manufacturer's requirements (RepairPal only)
  7. Faulty engine computer (ECM/PCM) (OBD-Codes.com, RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Check the engine oil level and make sure the oil matches your owner's manual spec; RepairPal lists low or wrong oil as common causes and an oil and filter change as the common repair.
  2. Find the bank 1 exhaust valve control solenoid, usually on the exhaust side of the cylinder head on the cylinder 1 side, and look for unplugged connectors, corrosion or loose wires.
  3. OBD-Codes.com notes the solenoid can be switched on its power side or its ground side depending on the vehicle, so check a factory wiring diagram to see how your circuit works before testing.
  4. With the solenoid and computer connectors unplugged, OBD-Codes.com describes checking each solenoid wire end to end with a multimeter on the ohms setting. Resistance should be around 1 ohm or less; an over-limit reading may mean an open wire, a loose connection or a bad terminal.
  5. To test the solenoid itself, measure resistance across its two terminals and compare it with the factory spec or a known good solenoid. OBD-Codes.com says an over-limit or excessive reading means the solenoid is likely faulty, and low resistance from a terminal to ground may mean it is shorted internally.
  6. To check for a short to power, OBD-Codes.com says to unplug the computer connector, set the meter to volts, put the negative lead on ground and the positive lead on each solenoid wire; voltage present may mean a short to power. For a short to ground, put the positive lead on the battery positive and the negative lead on each solenoid wire; voltage present may mean a short to ground. Unplug connectors and work back toward the solenoid to find either one.
  7. If wiring and solenoid test good, OBD-Codes.com suggests comparing the duty cycle commanded on an advanced scan tool with the actual signal at the solenoid using an oscilloscope or graphing multimeter at different engine speeds and loads. No signal from the computer may mean a computer fault.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The engine won't start or runs poorly along with the code
  • You don't have a wiring diagram, multimeter or a scan tool that shows solenoid duty cycle
  • The code returns after an oil change and a wiring repair or new solenoid
  • Other camshaft timing codes such as P0027, P0014 or P0011 are also set
  • Tests point to the engine computer, which may need programming to your vehicle

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0078?

Only carefully and for a short time. RepairPal rates it Urgent and says the engine may lose performance or even fail to start, while OBD-Codes.com lists poor acceleration and lower fuel economy. Check the oil level first and get it diagnosed soon.

What is the exhaust valve control solenoid?

It is the oil control valve for exhaust camshaft timing. RepairPal says it uses oil pressure and an electrical solenoid so the PCM can adjust when the exhaust valves open and close, and OBD-Codes.com says it is usually on the exhaust manifold side of the cylinder head.

Can low oil cause P0078?

RepairPal lists low engine oil and oil that doesn't meet the manufacturer's requirements as common causes, because the timing solenoid works with engine oil pressure. OBD-Codes.com treats P0078 as an electrical fault, so check both the oil and the wiring.

How much does it cost to fix P0078?

Neither source gives a repair cost for P0078. It can be as simple as an oil change or a loose connector, or it may need a new solenoid, so diagnose it before buying parts.

Sources

  1. OBD-Codes.com: P0078 Exhaust Valve Control Solenoid Circuit (Bank 1)
  2. RepairPal: P0078 Exhaust Valve Control Solenoid Circuit (rated Urgent)

Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.