I’ve had this watch for about 3 years. A few days back I found that my Casio Protrek Triple Sensor PRG-90T-7V displaying “op en” instead of the date. None of the buttons are working, only the Auto EL feature is working.
Here is what I do to fix it:
The PRG-90T-7V (probably all the Casio PRG series) has a couple of little springs inside that make contact with the back plate. If those springs are missing, then the watch thinks that the back plate is missing. It also display the same error if the backplate removed.
*** I am sorry for the quality of the pictures here, they all taken with my cellphone. I have no camera ready at the time of this work. ***
Here is the picture of my PRG-90T-7V showing “op en”.
I do not recommend that you try to fix it yourself. You should send it to Casio for a service. I do this because there is no Casio service center in my area and sending it to another nearest service center will take days.
[*] Find a small (+) screw driver before you begin.
[*] Remove the band. You have to do this before you can get the backplate off. Be careful with the claps that holding the band. This can be difficult because you need to disengaged two spring bars simultaneusly. Be patients and slow, the spring bars tend to fly when you remove the clasp. Do not lose the them.
[*] Unscrew the sensor part. See the picture below.
There is two screws there.
[*] Then you need to remove the plastic/resin that obstructing you from getting the back off.
You need to pull them carefully especially the one near the sensor. I cannot get this one off, because there is some part of it that stuck under black part which has “TOUGH SOLAR” text.
Here is how to pull them off. Be gently. Follow the yellow arrow to pull them.
Here is how it looks after pulled off, notice the one that still there — the part marked with blue [A]. While the other one [B] is pulled off. There are 4 screws there (blue circles), unscrew them off gently. Push the backplate with your finger while unscrewing to prevent the springs inside flying off while you pull the backplate off.
[*] Here is the backplate after pulled off.
[*] Here is the watch without the backplate.
There are 3 springs there, red circles. The two which close to each other is the backplate detector.
The black rubber ring showing by the blue arrows is the seal that helps makes the watch water resistant.
[*] This is the inside of the backplate.
There is a small plate (yellow circle) glued to the backplate which is not where it should be. The heat here in my city probably the cause of it moves. I had to push the small plate upward as shows by the arrow. Once it back to the right position, test it by putting the backplate back (without screwing) to see the watch if “op en” error dissapear. If not, move the small plate where you think the right position is. It should be on top of the two springs which close to each other.
[*] If the error dissapear, put back the backplate, screw in. Put back the plastic/resin carefully. Screw the sensor part. Lastly, put back the band.
[*] Here is the picture of the watch after fixed. The date is back, YAY!