The sensor is at the left rear of the engine on top of the engine oil cooler, they get plugged with hard water I have had to replace mine twice.

pull the oil cooler off and make sure it's not clogged in any way, seen all kinds of crap stuck there

Ok this is my next step then

I will flush the rear oil cooler with a hose and see what comes out. Then I will switch the sensors from the engines to see if one is bad

You need to be able to look through the cooler holes to confirm there are no small stones, wood chips, etc lodged in the holes. Water will generally not work for proper flushing. ...either remove the cooler or do this while mounted. I use a piece of straightened coat hanger (or welding rod) to push pebbles out. A few times, I've had to remove the cooler and use quite a bit of force to get the stones or wood chips out - due to the location of the coolers on my 575's - they are a b!tch to get to.

On more than a few Bajas, I have seen issues with the fresh water flush system getting gummed up with debris and causing water flow/pressure issues.
There is a spring loaded ball check valve in the Tee into the seawater side of the system. Remove your "T" fittings and pull this ball out of there. It's not needed.
This may or may not be your issue; but it could be an issue in the future too.

Also, I get a little spooked on switching sensors like this from motor to motor. I get the troubleshooting aspect of this, and ease to pinpoint problems. I myself have done this at times. But when it comes to sensors that screw into blocks and other parts, you never know when one is going to go sideways on you, and snap off or worse. It may be worth your while to just replace the sensor, vs. trying to swap them.

I bought a new sensor. im going to just replace it. looking at pictures it looks like this rear oil cooler is a ***** to get to!! Any advise?

I made these just as guildlines for me tonight when im laying over the engine.

Any chance you have a picture of where this T fitting is?

flushing may not work, had one a while back that was full of tiny rocks embedded in the screen that would not flush out, had to be poked out with wire..

I guess I will try flushing it first. I don't know whats involved with removal