The Dreaded P0688 Code

[Shu232]

My coil pack harness melted and Ii had the entire harness done back to the ECU. Now the car runs smooth, but I'm still getting the P0688 code and no blown #29 fuse.
Any thoughts?

[walky_talky20]

Fuse #29 drives the 4 coils and a particular pin on the ECU (pin 121). That pin is how the ECU knows to set the code when fuse #29 blows. It is there to monitor the power delivery to the coil packs. Perhaps in the re-wire process, that wire to the ECU did not get connected back up. That's my guess, anyway. I'm not certain where it ties into that circuit, but I would assume there is a junction of sorts somewhere in between the back of fuse #29 and coil pack #4.

If there is no eureka moment ("Oh THAT wire!"), then I would probably test that theory before going "full ham sandwich" on a wiring investigation. Turn the key on, then check the coil packs for power. Should be all good. Then go to the ECU and check pin #121 for power (back-probe). If coil packs are powered up, but pin #121 is not, then that wire is open somewhere. Check obvious places where modifications were made.