Genie GDO infrared sensor problem

My 3 year old Genie Garage Door Opener (screw type) will now only close if I hold the wall button down for the entire travel. It will open from the remote control but not close using it. This, combined with the fact that the right hand (looking from inside) infrared unit LED is very dim (always used to be bright, and blinked if obstruction present) leads me to believe it's that unit. The left IR unit has a solid bright LED. Wiring seems to be fine. It's been really cold this winter, I suspect frost or cold damage.
My questions: - Anybody have experience with this kind of problem? Diagnostic suggestions? I can't find the owners manual (of course). I have rudimentary electronics knowledge (volt meter stuff, nothing fancy). - Two home centers near me (you know the ones) offer everything but replacement IR units. Any suggestions for on-line sources to consider (or to definitely avoid?) - I gather defeating the unit is a bad idea or not practical. My young- est child is 19 but, granted, it might save my car trunk someday.
Thanks in advance for reading this and making any suggestions!
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Garage door openers got along fine without "sensors" for 30 or 40 years before the Federal nannys decided you were too stupid to be trusted. Besides, they won't save the paint on your car trunk lid...only keep your 19 year old from being crushed to death.

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wrote:

Start here: http://www.geniecompany.com/Support/gdobasictrouble.htm
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Andy,
http://www.geniecompany.com/Support/gdobasictrouble.htm#workswallcontrol Genie Photo Cell Troubleshooting
The following is a list of things to help troubleshoot the photocell circuit and order parts if necessary:
1.) Unplug your opener & plug it back in. This action may reset them.
2.) Using a screwdriver, "short circuit" the two screws on the red light photocell together. This may reset them.
3.) Inspect the wires running from the opener to the photocells for breaks or loose connections.
4.) Clean the photo cell lenses of dirt, cobwebs, etc.
5.) Unscrew the wires to the photocells at the opener, remove the photocell modules from their brackets and wire them together with a short set of wires, approximately one foot long, directly to the opener. If they work correctly replace the existing photocell wires. If they fail, check for voltage (12 volts DC) across the photocell terminals with a multi-meter on the DC scale. If power exist, replace the photocell modules. If no power exist replace the main sequencer board in the operator.
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You will find replacement parts for your Genie here: http://www.new-garage-door-parts.com/genie.html

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