Peli

Two months ago, I purchased a dealer maintained 1997 LS400 with 220k miles. This last week, the transmission started slipping without the check-engine-light coming on. At the same time, the A/C stopped blowing cold, and the power locks don't work consistently. All the fluid levels are fine, though the ATF looks a bit worn. The transmission problems seem to occur once the car has warmed up. Sometimes, there's little power when reversing, and the car seems to be unable to shift up for highway speeds.

How likely is this being caused by the ECU's capacitors going bad? Again, the CEL doesn't come on.

dicer

Sounds like someone using a one size fits all automatic transmission fluid problem and goofing up seals and friction materials.

Superfast1

Most likely, the shift solenoid E inside your tranny is going bad.

Peli

Would that trigger the check-engine-light or not?

Moarpower

The first thing I'd so is service the trans and ensure you are only using the correct fluid. Other than that it sounds like a transmission problem. A slipping trans is coming from the trans. If it's just not shifting at the right time it could be elsewhere.

as the car warms up the fluids warm up and lower in viscosity. Change the fluid. If it continues there is likely some damage done already.

Losiracer2

replace the fluid with genuine Toyota T4 fluid and change the filter out to see if that helps. It helps my car produce clean crisp, quick shifts in PWR mode now and I've got 284k on my 97. I drained and filled it twice and then on the third time I replaced the filter and called it good.

Yamae

Since your ATF is a bit worn, it may be a time to change the ATF and the strainer. Also check the ripple voltage as is written in the FAQ page if you think it's the ECU issue. I'd say, "Two decades are certainly long enough for those QAS capacitors to fail". The leaked QAS liquid often damages traces badly when too late.

Regarding the power lock issue, it's totally in another world. You need to check the MPX body/door computers and the door lock actuators. Those are nothing to do with the ECU.