Had this code randomly happen today with my 03 2.4 L. Spent a good chunk of time reading forums and sifting the FSM, and I think I have found some information that will be helpful to anyone that has this issue.
First, the specific error code P1491 is defined as a "Radiator Fan Relay Circuit Condition" based off the scanner I used with my phone. From what I have read, there are multiple reasons for this code to be thrown. The first could be a blown fuse. First check the 40A fuse in slot 4 of the power distribution center (PDC). (Ref fig 1 and fig 2).
Further looking at Figure 2, from this print it shows that these fuses are always hot, as the battery is directly connected. I confirmed this when I skipped the relay to test radiator fan functionality without the key in the ignition. In order to test to make sure the radiator fan was functioning, I used figure 3 to figure out which wires needed to be connected.
Now that I knew which wires I needed to connect to skip the relay, I had to locate where the actual radiator fan motor relay was. This is totally dependent on which year you have, in an 03 2.4L it is located on the side of the PDC, as seen in figure 4 below.
After locating the relay and disconnecting the adapter, you need to jump the blue/pink wire to the yellow wire on the adapter itself, as seen in figure 5.
Note the blue/pink wire is HOT, use the correct gauge wire to avoid melting and burning your fingertips off . For me, when I jumped these wires my fan spun, so I know it is not a radiator fan issue. If it does not spin, either the 30 Amp Circuit breaker, seen in figure 6, is blown, the fan has an issue, or you are not getting any power to the adapter itself. As far as I am aware, the only fuse that would prevent this, assuming there is no loss of any other primary function in the vehicle, is that in Figure 1 as previously discussed.
For where I am at now, I'm assuming my radiator fan motor relay is bad, which can be found here: Radiator Fan Motor Relay
As for the things I am unsure of, I do not know if there is a fuse in the glove box that can be a culprit to this. I could not find in the FSM a fuse in the fuse box that was tied to the PCD. I also tried seeing if there was a fuse tied to the PCM, specifically module C2, seen in figure 3, which I found nothing. In figure 3 it is also somewhat confusing that the module has an input from the "high speed radiator relay", but there is no specific component in the FSM or parts list called this.
I am fairly certain that the "input" from the high speed radiator relay that actuates the radiator fan motor relay is temperature driven, specifically from the engine coolant temperature sensor. From looking at the Pin-outs, as seen in figure 7, there is no mention of the relationship however, so I am stuck as to where I can go. If I had to guess, whichever data is collected from the coolant temp sensor and gathered by C1, it is used via the PCM and sent as an input to the radiator fan motor relay.
If anyone has any additional info I, as well as anyone else having this issue, would greatly appreciate it. Let me know if I gave any misleading information as well. My hopes with spending the time of making a "mini guide" were that I can eventually turn this in for submission to be added to the "how-to-guide" section of the forum. (@Jerry Bransford @Chris @Wildman this will be reformatted to be more of a how-to guide once I get some answers)