I am a novice, but am having fun trying to learn. I have a 1991 F150 5.0L, v8, automatic. I was getting a code 41 & 51 on the Engine off test. I replaced the ECT sensor, and am now getting a code 21 on the same test. The truck is doing the same thing as before. it will start from cold, but after it warms up a bit, it won't start again. (before replacing the ECT sensor, I was getting a 98 & 51 on the engine running test). Thanks in advance for any help!!