New to me 2002 av. Now stuck in 3rd gear! Ugh

Hi.  I made it my whole life without owning a Chevy. With a growing family I decided the avalanche is a perfect fit. Found one locally that was a one owner Well maintained with 108k  and took the plunge.  SES light has been on since/when i  bought it due to what the owner said was a downstream stream cat converter O2 sensor that he provided new  for me to install.
Well, 50 miles later and transmission stuck in third gear.  ?
It seems that  this is a safe mode and is common to a few problems  that I've learned thanks to this fantastic forum. 
1: autostart installed incorrectly.  I do have a two way compustar autostart that had occasionally misread whether locked or unlocked and done a weird clicking thing after autostarting until key was turned on. 
2: bad ignition switch
3: tranny issues.

So I brought it to a shop to have the codes read and it came back with P0756.  Anyone  familiar with this code related to this problem? I was told by him that it may mean a plugged orifice in the tranny. Maybe he said port 29?  He also said there was something wrong reading about the upstream O2 sensor from the cat converter. But when he checked it closer  they both seemed to be sending signal accurately.  I had the included new downstream one so I installed that with no change. Anyways, he reset the computer and the truck worked  perfectly for 20 miles then back to stuck in third in drive. If I shift down to one or two I get second gear.
My question is how do I move forward? I have to drive 600 miles to get a camper in three short weeks.
Should I take it to  A transmission shop and have them pull the pan and look into the port from p0756?
Should I take it to a place that services this brand of autostart and tell them it was installed incorrectly?
Should I throw parts at it and install an ignition switch?

Please help!  I want to love this truck!

http://www.howstuffinmycarworks.com/GM_truck_bad_ign.html

http://www.chevyavalanchefanclub.com/cafcna/index.php?topic=64603.0;wap2

Oh ya, gauges are also misreading when stuck in 3rd.

One of the links I got when I Googled your code:

http://www.autocodes.com/p0756_chevrolet.html

Hey thanks.  That's really helpful.  Seems the answer is always right under my nose. 
Im pretty handy with a wrench, i have to be since my daily driver is a vw bug, you think this is something i could tackle in my driveway?

I kind of think you should take it to a tranny shop it could be the valve or it could be a blocked passage. Trannys are quite complexed

Can anyone confirm that the two shift solenoids are the exact same?  It seems locally i can only find a a place with two 1-2 A solenoids.  Im in alaska and could order online but lokking to do this work asap. 

According to the parts site I see yes the 1-2, 2-3 solenoids are the same part number.


Edit. on the 4l60e only.



What area in Alaska are you at, we have members up your way.

Thanks. 
Silly question, but how can i tell my tranny type?  Can't find it in paperwork anywhere. 
2002 av, 5.3, z71, auto with tow/haul (that has never worked!. Connected to this problem i assume?)

Im in Anchorage

Unless it's been swapped with something else it will be the 4L60e.

Yes, those two solenoids are the same part number.  If dropping the pan to get to the underside of the valvebody doesn't intimidate you then changing these should be very doable in your driveway.

I see enoniam answered already, but I'll post anyway
If no one has changed it, it came with a 4l60e or 4l65e


(1) (2 = 2002)
(2) Model
(3) Hydra-Matic 4L60-E
(4) Julian Date (or Day of the Year)
(5) Shift Built (A, B, J = First Shift; C, H, W = Second Shift)
(6) Serial Number
(7) Transmission ID Location
(7) Transmission ID Location
(8 ) Optional Transmission ID Location

Not sure about the tow/haul, could be a couple of things

You are all very helpful.  I got all my parts and will tackle it soon.
I got two solenoids for 1-2 and 2-3 replacement.  Should i do 3-4 while im in there? Doesn't seem to commonly need replacing like these two do. 

You are thinking like me, if one is bad you may as well replace them all.

But in the manual there is no 3-4 but there is a 3-2

(36) Vehicle Speed Sensor (VSS)
(66) Torque Converter Clutch (TCC) Solenoid Valve
(69) Automatic Transmission Fluid Pressure (TFP) Manual Valve Position Switch
(367a) 1-2 Shift Solenoid (SS) Valve
(367b) 2-3 Shift Solenoid (SS) Valve
(377) Pressure Control (PC) Solenoid Valve
(394) 3-2 Shift Solenoid (SS) Valve Assembly
(396) Torque Converter Clutch Pulse Width Modulation (TCC PWM) Solenoid Valve

Gotcha.  But hold the boat! Would you folks agree with this statement i found in another thread:
When you have tranny shift problems AND gauge issues as well as Check Engine Light. I would say I am 99.999999999999999999999999999% sure you have a bad ignition switch.

I have those problems.  And i have my receipts and would much rather do an ignition over dirty tranny work, but def don't wanna do both!!

I'm no tranny expert I'm replying to what you post. It's worth a shot to replace the ignition switch first.

It is proabbly best to proper diaganose the issue before changing parts.:

TO test Ignition switch:
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,64603.0.html

True.
  All ignition fuses had power with key in ON.  So...that's that.  Onto the solenoids. 

Ok.  Done.  What a PITA to get the pan down around the shift lever and header. 
So the dude at the dealer counter sold me 15 quarts cuz i "need 14.4", he said.  After 8, i checked the dipstick and it's way high! 
How much do I actually need?
No way im taking that pan off again.  Any way to get some out?

More than half is stuck in the torque converter.

To get some of it out get a fluid transfer pump to stuck it out through the dipstick tube.

Gotcha.  What about draining from this trans cooler line i found in another thread?  Seems like a good option to simply dump a few quarts. 
Should i drive a bit to get levels squared away or is that a no no?

If you have a tranny cooler in front of the radiator there is not a easy way unless you use some type of pressure, say a like an air compressor.

You could suck some of the fluid out to the normal level start the AV put it drive then drop at down though each gear to low then back up to reverse then check the level again. Let it stay in each gear for 15 seconds.


I would not drive it till it gets hot then check the level again.

Oh yeah did you happen to change the filter while you had it apart

If you have a 2-wheel drive your best bet may be to pull the driveshaft out the back and let fluid drain from there.  Should drain to somewhat below the fill level.

If not a 2-wheel drive I wouldn't recommend trying to pull the t-case to do this because that would make it harder than alternatives.

If 4-wheel drive pulling the tranny fluid hose going into the radiator and firing the engine up shortly should result in a good deal of fluid exiting in a short period of time.  I'm not sure off-hand which hose into the radiator is the one pushing the fluid into the hose.

If it were me I'd just loosen the pan bolts enough to get the fluid to start seeping out and after a quart or so is out retighten.

Like MS03 said, start it up and with it standing still run the gear lever through all of the gears.  Since you didn't have the tranny apart beyond dropping the pan there shouldn't be too much of a difference before and after doing that but it doesn't hurt to do it anyway.

Thanks.  Yes I changed the filter.  It is 4x4 2002 z71 1500, so I believe it came with a trans cooler but im gonna have to look today.  If so i will try that way.  If not, now that ive had a good nights rest separating me from the hell of pan pulling that was yesterday, I suppose i could loosen the bolts and dump some out that way. 

So just to finalize, the solenoid replacement fixed my problem.
I used a handpump through the dipstick and pulled out two quarts and she drives and shifts like a champ.  Hopefully that concludes my introduction to wrenching of Avali.
Now, on to modding!