Drivable short-term, but fix now to avoid damage.
This code means shift solenoid C is stuck in the on position, so it keeps directing fluid pressure even when it shouldn't, disrupting normal gear changes. Common causes include a stuck or failed solenoid, debris in the valve body, or low or dirty transmission fluid. Repairs usually involve a fluid service, cleaning or replacing the solenoid, and many cases don't require rebuilding the transmission.
$150 – $650
Varies by vehicle and root cause.
A short trip is usually fine, but don't rely on it. If the transmission holds a gear or drops into limp mode, the car becomes harder to drive safely, so get it diagnosed soon.
Repairs generally fall between $150 and $650. A fluid service or cleaning is cheaper, while replacing the solenoid raises the cost depending on your vehicle.
Yes, it deserves prompt attention. A solenoid stuck on disrupts shifting and can stress the transmission, so it's best treated as a high-priority fix.
Debris, varnish from old fluid, or an internal fault can keep the solenoid energized so it won't release. That keeps fluid flowing when it shouldn't and triggers P0766.