EdenaCycle

I'll try to be short & sweet yet accurate while I bombard the internet with this question in the hope of quickly isolating the issue.

My wife's had a '94 Lexus ES300 that we've done some general wear & tear maintenance on in the few months she's had it--new thermostat, new brake lines, etc. Awhile back, check engine light and O/D Off light started popping, intermittently at first, before the car completely shut down on her way to work one day. Had it towed back, checked it on the OBD II--P0753, shift solenoid A. Complete teardown and overhaul of the valve body ensued; replaced solenoids A and B, cleaned and replaced all the valve body gaskets, seals and O-rings, and replaced the transmission fluid--this is only important because other corners of the internet have suggested that dirty transmission fluid or an issue with the linkage could be at fault for what comes next.

Her daily commute is thirty miles, give or take. Ran perfectly fine first time taking it back to work on Friday, but the lights came back on the return trip twice; first time she noticed they'd gone off after a stop, but they were back on by the time she got back home, and the car wouldn't go into low gear, which was the same issue as before. Check the code and it's popping P0753 again. Friend of mine (who did the work, I should mention) with a background in race garages (Comp Coupe Vipers, Miata Spec, others) and electrical engineering thinks it could be an issue with a harness or a ground somewhere that's making it throw the code. Since we've got the internet at our disposal, it makes sense to check and see if anyone else has run into this issue before he gets into full gremlin-hunting mode--especially since he's also working on an engine swap for a project of his and I'm not trying to eat all of his free time.

LeX2K

Bad ECU? The caps on these cars are known to leak and cause all sorts of issues any other problems with how the engine runs? Is the car rusty? Did you use Toyota parts or generic.

EdenaCycle

Of course I'd forget to mention that--yeah, it's not quite rust bucket status, but it is rusty. My friend had suggested that as a potential issue with it grounding out somewhere in the system, so that's probably the culprit.

ES300PLT

Curious, did you replace the Solenoid A and B with a NEW Solenoid or a USED solenoid? Did you check the resistance before and after to see the difference? Also you should reset your Gearbox ECU. I think you can disconnect a few fuses to reset it and then see if that helps. Otherwise unfortunately, you will have to open it back up and check the Solenoid again for the resistance range, and possibly your wiring/wiring loom to make sure. Where did you get the solenoid part from?