JD 155c mows in reverse but not in forward. Changed RIO switch but still have problem. Any idea which switch to check next?

The wiring diagram shows the reverse switch is normally closed until activated. This would mean forward travel requires switch closed else PTO disengages... same as your experience.

From your description of progressive failure I suspect the switch has failed or has come loose and moved from it's normal position.

Go to the JD website below and search "reverse switch". A parts diagram should display. Look for item 15, part number LVA13307

https://partscatalog.deere.com/jdrc/navigation/equipment/61166

The reverse pedal or linkage must somehow activate the switch.

There could be a problem with the PTO switch or safety interlock module or electrical interconnections so if the reverse switch checks out, further troubleshooting will be necessary. Start at the reverse switch and work your way to the PTO switch then the interlockmodule.

Reverse With Blade in Operation
Step 1
Come to a complete stop.

Step 2
Locate the RIO button on the controls area beneath the steering wheel.

Step 3
Press in on the RIO button and hold it down. Then press down on the reverse foot pedal, located on the right foot rest, with your right foot.

Step 4
Release the RIO button and drive the mower in reverse.

Step 5
Repeat the above steps each time you want to reverse with the mower blade engaged.

Here's the link and I'm sure you will get the solution of your problem.
John Deere 102 115 125 135 145 155C 190C Lawn Garden Tractor Technical...

Check if lever come unhooked or a stick or something blocking it.

With everything you said I never heard you say you checked the oil or put in oil Hope it didn't run tight on you.I'm not sure if your model has one but some also have safety switches under the clutch pedal . If not have you checked the hydrostatic oil im not sure if your is hydro but that could also keep it from shifting in gear and possibly keep it from starting to protect transmission. Just some ideas good luck

Many machines have a micro switch on the shift lever, it is to stop you mowing in reverse, maybe this switch is sticking, some machines have a position on the ignition switch to turn this function off.

All riding mowers have a seat safety, blade engage safety and a brake clutch safety, some mowers have a reverse mowing safety. Seat under the seat, blade and brake you will have to check the linkages underneath, and if there is a reverse mowing one I have seen them near the shifter handle right under the fender.

Yes there is a safety switch.And this safety switch has to be engaged.If its not engaged then the JD tractor will not start. On JD 855 the safty switch is located under the seat, There is a rod that must be depressed before engage Somehow that rod is bent or its not pressing the switch to the point that the seat would not fully engage the safty switch.In some cases the switch itself is faulty.It has to be checked . Thanks. keep updated for any more query.you can rate this solution and show your appreciation.

Check the belt that runs from the engine to the transaxle, its separate from the belt that turns the blades. If the belt is ok I recommend you take the mower to a JD dealer.

Either you need a seat switch or the reverse switch has failed.If the deck will engage while you are pushing down the forward pedal then the rio or reverse switch is not working or it's out of adjustment.If you can not get the deck to engage at all then the problem is likely the seat switch.