04 Sienna LE P0793 Hard Shifting

I have a 2004 Sienna LE with 169,500 miles. It started to shift hard between 2nd and 3rd gear in traffic (it had previously never had ANY hard shifting). A few minutes later the CEL came on along with the VSC and Traction Control lights. Both Advance Auto and a Toyota dealer confirmed that the codes showed the speed sensors on the rear wheels (both sides) were bad. The Toyota dealer also declared that the low gear solenoid in the transmission was bad and the transmission would need to be replaced soon. I found it strange that the transmission started shifting hard at the exact same time the speed sensors went bad. Is it possible that simply replacing the speed sensors would fix my hard shifting issue? I am a shade tree mechanic that does brake work and other intermediate tasks. Is speed sensor replacement an easy job on this Sienna?

Do you know how the Toyota dealer diagnosed a bad solenoid? The thing is that the solenoids do what they are commanded to do by the PCM (I’m assuming TC and EC are integrated on the Sienna), and the PCM does that according to data it gets from things like speed sensors.

It’s odd too that the P0793 code is actually about the transmission speed sensors (I think there are two - an input and output).

And it’s even more odd that you could get TWO wheel speed sensor problems at the same time, AND an error code for speed sensors on the transmission, AND a report of a shift solenoid problem - all that the same time?

I smell two things: 1) a Toyota dealer where people use computers more than brains to actually look at a problem holistically, and 2) a damaged wire(s) or bad ground someplace.

Thanks cigroller. It might have been P0794 (intermittent) plus P2746. I am dubious that they could diagnose the solenoid in the transmission with just a test drive, particularly with at least one bad sensor (potentially) in the mix. You noticed the oddity of TWO speed sensors failing simultaneously and that caught my attention as well. I will have my hometown dealer look at it next week. I trust them far more than the place I visited in Miami (where they were very nice and only charged me $70 for the 2.5 hours they spent trying to diagnose my problem).

Any “intermittent” code makes me smell wiring problem even more. I’ve never been under the hood of a Sienna, but these speed sensors are probably pretty easy to inspect in terms of wiring harnesses. I like to at least look for the obvious before dumping another pile of $$ on diagnostics. If one of the sensors turns out to be a problem, they also often just screw in and out or are held in by a single clamping bolt - easy stuff.

I now have the three codes that were pulled. P0793, P2238, and P0776. Dealer says the O2 sensor code can be ignored since it is being generated as a by-product of the speed sensor code. I have dealer #2 looking at it now and will keep everyone posted.

Take the van to an independent transmission specialist for a second or third opinion. These faults are not common on this vehicle. Look for signs of invalid data coming from the PCM such as implausible speed. possible faulty PCM.

Here is the update. The second Toyota dealer found that the wiring leading to the speed sensor on the transmission was corroded and frayed. They fixed the wiring to the sensor and all warning lights are off and the transmission shifts flawlessly. Cigroller was dead on with his assessment of the bad wire. As it turns out, the first shop was a bunch of techs looking at codes, the second shop had a real mechanic who actually looked to see what was wrong by doing actual troubleshooting. $350 later I am happy to be getting out of Florida without a major repair!!!