Engine/Transmission (1998.5 - 2002) Check Engine Light Driving Me Nuts!

[Lube Man]

After killing my two batteries by leaving the ignition switch on I replaced them both only to have the check engine light come on reading code P1291 (intake air temp problem). I assume that trying to start with dead batteries did not allow the heaters to work properly and triggered the P1291. After erasing the code the truch ran fine for a week then the P1291 code came back. Erased it again and everything ok for a few days and then it comes back. The heater grids appear to be working ok as the truck starts right up when cold and runs normally. I can also see the dash lights dim as the heaters cycle on and off.



Any ideas what is sporadically triggering the P1291 code?



By the way, I bought an Actron OBDII code reader through e-Bay for $100 including shipping. Got tired of running down to Autozone who lends one out for free.

 

[rkressg]

You could have a problem with the IAT (Intake Air Temp) sensor. It is located just behind the MAP sensor on the drivers side of the block (way in the back). As with anything electronic, they can fail. At least they are not all that expensive (from a Cummins dealer), plus, you should see a fuel economy increase after changing this part of about 2 mpg.

-Rich

 

[Lube Man]

Yeah, I have seen where it is. You need real long arms to get to it. Why would killing the batteries cause the switch to malfunction? Also, please explain me more about why I would get a 2mpg increase.



Thanks, Ed

 

[rkressg]

I can not give you a direct answer as to why a failed battery would cause this, but electronic components fail (IAT). It could also be a problem with the relays (below and behind, just to the left, of the drivers side battery) on the grid heaters, but they seem to be cycling fine.



The IAT measures the intake air temp and sends a signal to the ECM which will attempt to adjust the lean/rich fuel mixture to optimize for emissions. A failing IAT tends to not send the correct signals, which will not optimize the engine operations. I will not guarentee a MPG increase, but it is a common trait seen in this forum. When truck owners change that part, that is a typical increase in economy seen here.

-Rich

 

[Lube Man]

Thanks Rich. One furhter question. I added an Edge EZ. Could that have anything to do with it?



Ed

 

[OTRPU]

I'm seeing some similarities here, couple days ago my Check Engine light came on, also I have an Edge Card, not sure if it's the EZ, it sits atop the fuse/relay box driver side to right of batt. Had codes read, MAP sensor voltage low. DOC cleared it, he ran it through diagnostics, code didn't return. Drove the truck 8 miles, shut it off. 3 hours later started the truck and the Check Engine light came on again. No operational symptoms that the sensor isn't working. DOC's baffled, thinks maybe the Edge is involved. Where's the cheapest place to get a MAP sensor? Will complete delivery in Petaluma, CA and have him diagnos on trip back.



Cheers,

Steve J

 

[Lube Man]

Is the IAT just used on startup or is it monitoring all the time? If only used on startup maybe it is not really needed.

 

[rkressg]

No, the IAT sensor monitors continually and allows the ECM to make minor adjustments to changing conditions (for emissions).



Two things come to mind with the EZ. I am not that familiar with the EZ (I use a TST box), but the EZ mignt not be transmitting the voltage back to the ECM, or the MAP could just have died. They are not all that expensive (from Cummins, but a rip off from Dodge).

 

[OTRPU]

Well, my MAP has resurrected, left the trailer at a friends house where the locals are more trailer tolerant. Drive home the Check Engine Light was on. Parked the truck for about 21 hrs, went back and hooked up the trailer, Check Engine Light went out. . . Suppose it went out cause I got new Steers? I like out, but just hate intermittent.



Cheers,

Steve J