Cummins ISL 400 121 fault code

[KeysEagle]

I have spent time and money at 2 different Cummins repair centers to repair the same problem. Check engine light comes on, diagnosis by both shops indicated a fault code 121 engine speed sensor or engine position sensor, one says the other was wrong. I had both sensors replaced, same check engine light comes on. Now I am told it is an intermittent problem perhaps with the ECM or wiring harness. I feel like I am getting the run around. Why does Cummins have a troubleshooting tree to follow that apparently does not repair the problem. Anyone else had a similar problem and any solutions short of replacing parts until the problem is solved?

[a k]

I just looked thru my INSITE codes for all 4 ISL's and can't find a fault code #121

[a k]

Here's a 121 code but it's for the ISM.