P3494 is set when the exhaust valve of cylinder 12 does not follow its command although the actuator circuit is electrically intact. No manufacturer documents this code publicly; J2012 reserves the codes for cylinders 9 to 12 for engines with more than eight cylinders. Valve-based shut-off traps a charge in the cylinder; if the exhaust valve keeps opening, that cushion leaks away, which a Ford patent proposes to detect through exhaust pressure, and if it sticks shut on reactivation, compressed gas loads the valve train. The Aventador V12 and Bentley W12 sidestep this by bank shut-off without published valve switching, and for the Mercedes M137 with its hydraulic solenoid pack no public code assignment is known. Electrical faults of this actuator: P3493, P3495, P3496; intake function: P3490.
Medium severity. Can I keep driving?Drive little and gently. If you hear timing chain noise or the oil light comes on, shut off the engine.
Cylinder 12 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine lamp on
- Jolt or knock on reactivation of cylinder 12
- Misfire on cylinder 12 (P0312)
Most likely causes
- Exhaust switching element at cylinder 12 sticking
- Oil pressure or quality insufficient for the exhaust switch
- Exhaust actuator sluggish
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It affects emissions.
What each character of P3494 means
- P: Powertrain: engine and transmission
- 3: Generic or manufacturer code, depending on the range
- 4: Emissions
- 94: Specific fault in that system
Frequently asked questions
Can I keep driving with code P3494?
Drive little and gently. If you hear timing chain noise or the oil light comes on, shut off the engine.
What causes code P3494?
The most common causes are: exhaust switching element at cylinder 12 sticking; oil pressure or quality insufficient for the exhaust switch; exhaust actuator sluggish.
Related codes
- P3493Cylinder 12 Exhaust Valve Control Circuit/Open
- P3495Cylinder 12 Exhaust Valve Control Circuit Low
- P3496Cylinder 12 Exhaust Valve Control Circuit High
- P3490Cylinder 12 Deactivation/Intake Valve Control Performance
- P0312Cylinder 12 Misfire Detected
Sources
- law.cornell.edu: 13 CCR 1968.2
- patents.google.com: en
- en.wikipedia.org: Mercedes Benz M137 engine
- newatlas.com: 24363
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.