P0135 & P0155 come up on startup

Helping my father-in-law with his late 90"s Ford. codes P0135 and P0155 came up, he changed the sensors on both banks and they still come up. What is strange is if he resets the code when the car is warmed up the codes do not reappear until the next time he starts the car after it cools off, then they pop up within a few seconds. Is it possible the engine temperature sensor is faulty, telling the ECM that the engine is already up to temp and the sensors should be switching?
Any confirmation or correction of my theory would be appreciated.

Those codes are for heated O2 sensors.

Check the heater circuits to the O2s, including the fuse(s).

Will your scanner monitor engine temp?

Good luck and keep us posted.

He brought the car over and we looked at it. I found out the codes pop back up when you restart the car after being off only a minute or two.
It's a 1997 Crown Victoria. We checked his owners manual and can't figure out which fuse is for the o2 sensors. My scanner doesn't monitor engine temp. I tried unplugging the computer engine temp sensor and the codes came up on start up just as before, with the addition of the code for the temp sensor of course.

i just had the same problem and it was because the 02 sensor I replaced was on the wrong side. if you go to a auto sore like advanced or autozone get the Haynes manual for your vehicle it will tell you where cylinder one is on the engine. The o2 sensor near cylinder one is 1-1 the opposite side is 1-2.

Where did he get the new O2 sensors? Aftermarket ones may be functional but fail the strict OBDII protocol, which means that if the sensors don't warm up and get the vehicle into closed loop fast enough they will set codes. If the sensors came from Advance or Autozone they probably gave you some bad ones. I would recommend he return them and get a refund, then get two new sensors from Carquest, Napa, or the Ford dealer.
The Haynes manual contains lots of good information written in a very easy to understand format. Its a bargain and one good thing you will find at Advance or Autozone.