My 85 F-150 4.9L I6 has the dreaded feedback carb and all the sensors on it. The only real problem I have run into so far since buying the truck is that it stalls when coming from higher engine speed to idle. I picked up the OBDI scan tool and first ran the KOEO test and got a fault code 23 (throttle position sensor signal out of range). I replaced the TPS and no codes show up on the KOEO.

On the KOER test the following codes came up:

12 - RPM Idle out of range/high
13 - RPM Idle out of range/low
41 - HEGO sensor voltage low/system lean
58 - Idle tracking switch circuit fault

Figuring the O2 sensor was a good place to start, I picked up a new one, got it installed and retested. This time:

12 - RPM Idle out of range/high
13 - RPM Idle out of range/low
42 - HEGO sensor voltage high/system rich
58 - Idle tracking switch circuit fault

I haven't been able to find out anything about the idle tracking switch circuit. What's going on here?

 

Was wondering if the codes listed are in the order received or as you arranged them?

 

The codes came out in that order.

 

I had stalling problems with my feedback carb ('86 4.9L) and feared the worst. Decided to go with a rebuilt feedback carb because that was the cheapest and easiest thing to try (other than swapping out sensor after sensor).

So far so good - running good with a smooth idle and no more stalling. Good luck.