2003 grand cherokee 4.0 6cyl auto trans sometimes won't shift into overdrive.

It is not going into neutral. Your overdrive band is fried on your transmission. Typical problem on these transmissions. Transmission will need to be completely disassembled to get to the overdrive band. Complete overhaul. Recommended when you read the owners manual to not drive these uphill for long periods of time without shifting to regular drive.

wow no engine size stated, the 5.2L run a special trans... so....
hopeless to help you much.
zj jeep with automatic.
it didnt lie, it told you to service it, so take it to a shop and let the work on this old dog..
stuck , now what does that mean.
the shift lever lever stuck, or its cable failing end to end.? or?
or shifts from 1 to 2 and hangs there.
or down shifts from 3 to 2 and and hangs there
DIY ? question is that it? you never said intentions.....
so we open the 93 ZJ book and what do we see.
ax-15 stick box, seems not.
the aw-4 auto , electronically shifted with TSM (scan it yet?)
and the 46RH auto. used on the 5.2L, has no TCM brain.
its a Hydramatic.
here are the OD words off the 46h page

Overdrive Control Switch
The overdrive control switch is located in the instrument
panel. In the On position, automatic shifts
into fourth gear overdrive will occur. In the Off position,
the switch overrides the powertrain control module
preventing a shift to overdrive fourth gear range.
The switch has an indicator light that illuminates
when overdrive is turned off. The switch also resets
when the ignition key is turned to the OFF position
so that the automatic overdrive feature is restored.
The use of fault codes is employed to help diagnose
the electronic components that operate the overdrive
unit and torque converter clutch.

P1491 Rad Fan Control Relay Circuit An open or shorted condition detected in the radiator fan control relay control circuit. This includes PWM solid state relays.P1495 Leak Detection Pump Solenoid Circuit An open or shorted condition detected in the Leak Detection Pump (LDP) solenoid circuit.P0753 (M) Trans 3-4 Shift Sol/Trans Relay Circuits An open or shorted condition detected in the overdrive solenoid control circuit or Trans Relay Circuit in JTEC RE transmissions. Was MIL code 45. P0753 (M) Trans 3-4 Shift Sol/Trans Relay Circuits An open or shorted condition detected in the transmission 2-4 shift solenoid circuit (4-speed auto. trans. only).P0136 (M) 1/2 O2 Sensor Heater Circuit Malfunction Oxygen sensor heater element malfunction.

Aside from getting into an accident and having your wiring harness severely damaged, the other likely suspect is your PCM. These codes are not related to each other and indicate a much larger problem.

Yes, the pressure lever cable will keep the trans from going into lockup. Nothing will happen if you can't shift to overdrive.--------------------------------------------------------------------------------
Diagnostic Interface

Drain fluid into a clean plastic bowl, check for brass cuttings or metal shavings in sunlight. If present get complete trans overhaul.

possible shift solinoid failure (overdrive shift solinoid)

Did you scope the TPS Sensor?

Did you use old trans sensors
That may have been your only problem

Just saying ,won't even guess,speed sensor maybe

sounds like a possible shift solenoid. If it's due for a fluid change, do that and add a container of lucas trans conditioner. Likely you have a drain plug on the pan so if the fluid was recently serviced, just drain a quart out and replace with the additive, then top off.
If the condition does not change within a week, likely you will need better diagnosis from a reputable trans shop.

There is a temperature sensor in the transmission to allow overdrive to work once the temp reaches a certain point. This is mainly to aid in getting the engine & trans to operating temperature, you could have a weak sensor or it could just be taking a while to warm up. Sensor is accessible without removing the trans.

good luck

sounds like the downshift cable is out of adjustment, or stretched out. the downshift cable goes to the transmission. CARL,