Hey,

Over the past two days I have had the check engine light in my truck come on and off and noticed the idle on the truck being a little bit variable. I should first state that I have a 1989 Ford F-150 with a 4.9L I6 that turned over 144k miles earlier today. The first time the check engine light came on was yesterday after about 20 minutes of driving down the highway. I drove for about another 20 minutes with the check engine light on until I got home and then proceeded to see what code was corresponding to the check engine light. Doing a KOEO test it pulled code 51 which corresponds to "ECT sensor signal is greater than the Self-Test maximum of 4.6 volts." Due to it being very cold out I chose not to do much troubleshooting and just went to the local NAPA and replaced the ECT. This seemed to work at first until about 20 minutes into my drive today when the check engine light came back on. I was about 5 minutes away from work at that point so I just continued on. Four hours later I went to start my truck for the ride home and the check engine light came on right away. About 5 minutes into my commute home the check engine light went off and the truck seemed to run fine. I pulled the code when I got home tonight and again I got code 51. So my question is what would be the next step in diagnosing this problem?

Summary ...
1989 Ford F-150 4.9L I6 144k miles
1 - Check engine light comes on after 20 minutes of driving, code 51
2 - Replaced ECT with new unit from NAPA
3 - Check engine light comes on after 20 minutes of driving
4 - Check engine light comes on right after starting.
5 - Pulled engine code, code 51.
6 - What now?

Thanks in advance.