P0868 (Line Pressure Low) can be caused by something simple like low fluid level, or a loose / cracked / misinstalled filter. After that, likely a sticky main regulator valve (in the pump) or a worn pump. Could also be a bad line pressure control solenoid (part of the solenoid module), or bad wiring controlling it, or a bad TCM. Could also be a bad line pressure sensor (or associated wiring). So there are lots of possibilities!
First step is to confirm whether the "low LP" reading is accurate (and still present). Take it to a dealer or shop that has a scan tool, LP adaptor tool, and a mechanical pressure gauge. The LP adaptor tool is something you can plug into the trans (where the LP sensor normally mounts) that allows you to tee in a mechanical pressure gauge (and also leave the LP sensor hooked up). Check the LP at idle in Park. See if the actual pressure on the gauge matches the reading from the LP sensor. That will tell you whether the LP sensor is giving a correct reading. If not, first try simply disconnecting and reconnecting the sensor harness, making sure it is properly seated and latched. If the LP sensor reading is still bad, then you have either a bad sensor, bad wiring, or a bad TCM (that can't interpret the sensor voltage properly). Dealers have a Transmission Simulator tool (that simulates a "good" transmission) that will allow them to figure out whether the problem is the sensor or the wiring/TCM.
If the sensor reading is OK (matches the mechanical gauge), next question is: Does the actual LP match the Desired Line Pressure (DLP) value? DLP will vary with throttle, and during shifts. So the key question is whether the actual LP is following DLP properly. You cannot judge whether the LP is "OK" based on its absolute level. You must compare it to DLP. For example, LP of 60 psi is "OK" if DLP is also 60 psi, but if DLP = 120 psi then an LP reading of 60 is "bad" (low). Now you should note that the LP will often be below DLP at idle (especially hot), since the pump speed is low, but this by itself is NOT a problem. If LP is low at idle, bump the engine speed up to about 1200 RPM and check it again. LP should be "OK" at engine speeds of 1200 RPM or higher. Also, note whether the LP is stable, or is jumping around all over the place. Normally, the LP reading will fluctuate within a few psi of the DLP value. But if DLP is stable, then the LP reading should be stable also.
If the LP is unstable (jumping around 10 psi or more, when engine speed and DLP are constant), then I'd suspect low oil level, or a bad filter. So check the level, then drop the pan and look for a bad filter. Make sure the main (flat) sump filter is correctly installed, with the plastic snout pushed up THROUGH the seal in the pump housing (not just resting against it), and make sure the metal flange on the seal is FLUSH against the pump casting all the way around (not tilted). Check that the filter is not split open along the side seam, and that the base of the plastic snout is not cracked. Also, check that the spin-on (cooler return) filter is snug (not loose). It should be HAND TIGHTENED only; if you overtighten it you can crack the threaded plastic snout. Spin it on (like a conventional engine oil filter) until the gasket touches the case face, then tighten by hand an additional 1/2 to 3/4 turn.
If LP is stable, but low (even at ~1200 RPM), then you probably have a pump problem (stuck main regulator valve), or a bad control system (bad TCM). Note, however, that 30 psi is normally the minimum LP sensor reading (it will read 30 psi even if actual pressure is zero), so a constant 30 psi reading could indicate a completely disengaged filter. If you find a "real" low reading (say, 42 psi), see if LP goes up and down as you raise and lower the engine speed. If so, then you almost surely have a stuck main regulator valve (which requires pulling the trans and replacing the pump assy). If you find a stable low reading, that does not change significantly with engine speed, try disconnecting the main 23-pin electrical harness on the trans (while the engine is running), and watch the pressure reading on your mechanical gauge (the LP sensor reading will "freeze" when you pull the harness). If the pressure jumps up significantly (like from 50 to 140 psi) when you pull the harness, then you apparently have a bad line pressure control circuit (bad TCM or bad wiring controlling the line pressure control solenoid).
Hope this helps, and I hope (for your sake) that the problem is a bad filter and not a bad pump!
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