2003 protege O/D off blinking while CEL on from time to time
 

Hi guys, my Mazda 2003 Protege LX 2.0 automatic 4-door sedan has got the O/D off light blinking for months while CEL always on. I've searched all around but cannot find a consistent and clear clue. It got pretty much mileage (163k) so I wonder if I still need to invest (waste) money on repairing it. I recent got OBDII read which retrieves 3 error codes (P0090 PRC solenoid valve circuit malfunction, P0421 Warmup catalyst efficiency below normal, and P0715 A/T Input Shaft sensor circuit Malfunction) and I think the lights are caused by the P0715. At this point, I am not sure if the problem can be solved by replacing with a new transmission speed sensor (I found it only cost ~15 USD on ebay, such as https://www.carparts.com/details/Maz...E&gclsrc=aw.ds). But I am not sure if it is compatible to my car coz I also found other parts with different shapes all claiming they are compatible to my car.

The problem in my car is very interesting which I never found any similar cases from the google searching. When the car is cold, no problem, when I drove it for 15 minutes (probably it was overheating but I read a normal temperature from the panel), the O/D off light started to blink. There is no issue as long as I keep low speed but when it was running at 4th (highest?) gear it will be forced to change back to the 3rd --- Seems everybody have the same issue --- but I found more interesting things:

In 99% of time, when I keep driving for 5-10 min with several stops or slowing down (to 2nd gear for example), the blinking light will be off, and off for the rest of driving no matter how far or how fast I was driving. But the next day, same problem and same recovery happen again through and through.

I also found that in cold days and high humidity (like rainy) days, it will allow me drive longer time without O/D off problem.

A full stop does not ease the problem but only temporarily turn it off but when I went back to 3rd gear it will blink again. Only after several full stops it will be turned off.

Till now I only feel very annoyed about the gear switching but nothing happened seriously (not affect other gears but only 4th).

Anyone knows what happens given the clues above? Should I let it be and drive as usual (mostly < 45 mph in city) or should I let dealer or mechanic check the tranny? Appreciate for all comments and suggestions! Thanks!

How much is a new sensor from Mazda?

Have you rung any car part shops for a price?

Thanks for replying. Our mechanic suggested us buy some from ebay and mostly it cost between 15 and 100 USD. I checked with Autozone and it costs 130. Haven't checked it with Mazda dealer and I think that's probably our last choice. My first thought is 4S dealers are usually more expensive for both parts and labor. Considering the mileage (163,000) and the drivability at this point, my choice could in the end be stopping further investing to this one.