So my buddy got an immaculate 98 model a couple weeks ago and we finally got it all tricked out. Noticed some oil seepage at head/block interface so he decide to go ahead and put a head gasket on it.

The day after completing the job the jeep got pretty warm and it threw a code, P1491. It looks like someone put stop leak in the cooling system so he went ahead and did a complete flush. No sign of temp issues now but the P1491 code keeps coming back. The weird thing is the fan is behaving exactly like it is supposed to....immediately comes on when the AC engages and comes on when temp is reached.

We checked grounds last night and all is well there from what we can tell. What has me scratching my head is what the fault condition logic is that trips the CEL. As far as I know the ECU sees a temp threshold and pulls the relay to ground or if the AC clutch energizes the relay goes to ground. Both of the conditions seem to be working so what could be triggering the code?

Is there another device in-line that we need to be checking? I have read about diodes and PWM modules but haven't found anything definitive as to what the actual hardware/logic is on a 98. All my documentation is for an 01.

 

the fan can be working, but maybe it is pulling to much amperage causing the code to be triggered

 

I didn't think about that but now that I do that would mean that the ECU is sensing current off the main power leads, not sure how that could happen.