P0964: Pressure Control Solenoid "B" Control Circuit/Open

P0964 reports an open circuit on the second pressure control solenoid, and the most useful thing to understand about it is not the word 'open' but the letter 'B'.

On nearly every transmission that has more than one pressure control solenoid, solenoid 'A' is the mainline unit. It sets the pressure the whole hydraulic system works from, so when it fails, everything the transmission does is affected at once — every shift, every engagement, forward and reverse. Solenoid 'B' almost never has that job. It is a second, subordinate regulator, and manufacturers use it for a specific purpose: apply pressure for one particular clutch pack, control of one shift element, or a dedicated circuit that only participates in part of the shift schedule.

That difference turns the code letter into a diagnostic shortcut. Before anything is unbolted, the letter tells you whether you are looking for a system problem or a component problem. A driver complaining that every gear change has gone wrong is describing an 'A' fault. A driver saying the car is completely normal except that one particular change is missing, or that it will not hold a specific gear, or that something happens only above a certain speed, is describing a 'B' fault, and the description narrows the search enormously before a tool is picked up.

There is an important caveat that stops the letter from being a universal answer, and it is the single most expensive thing to get wrong on this code. SAE assigns 'A', 'B' and 'C' as slot indices, not as names for particular parts. The standard says there is a second pressure control solenoid and gives it a letter; it does not say which physical solenoid in the valve body that is. Each manufacturer maps its own hardware into those slots. On one transmission 'B' may be the solenoid feeding the direct clutch; on another it may be the one that controls converter apply pressure; on a third it may serve a band. You cannot look at a solenoid pack, count along it and decide which one is 'B'. Guessing by position is how the wrong part gets replaced, and on a transmission where reaching the pack means dropping the pan and the valve body, replacing the wrong solenoid is an expensive way to learn the pin-out. Get the wiring diagram and the solenoid identification for that specific transmission and confirm which terminal in the case connector is the 'B' control circuit before ordering anything.

The 'open' part of the title is the module reporting that its intelligent output driver can find no load on the circuit. There is genuinely nothing there — a broken conductor, a terminal that has lost contact, or a coil that has failed open. Because the solenoid is selective, an open here often leaves the transmission in a perfectly usable state. The affected clutch simply does not receive its commanded pressure, and the module compensates by restricting the shift schedule to gears it can still deliver safely. A fair number of these codes are discovered at a service or an inspection rather than reported by the owner, because the car has been driving on a reduced set of ratios and the driver has adapted to it without registering anything as wrong.

That quietness is exactly why this code is rated seriously rather than as a minor fault. It is tempting to treat a selective failure as less important than a global one. In practice the opposite can be true: a mainline pressure fault affects everything and is unmissable, so it gets fixed. A clutch that is not receiving its commanded apply pressure and is being asked to carry torque anyway will slip, and a slipping clutch generates heat and friction material until it is destroyed. Whether a given vehicle protects itself by locking out the affected gear or lets the clutch struggle depends on the calibration, and that is not something to find out by continuing to drive it.