I have been throwing a P0121 code for the last month that has no effect on power output. No check engine light, but code is present. I reset my APPs tonight, but here is the deal.

I checked the white tag on the back of the sensor & it said .667vdc. I changed this sensor last summer & kept the old one, it says .595vdc. I checked voltage at the orange/blue tracer wire going to the pcm & got .05vdc. I adjusted my APPS until it read a voltage of .667vdc on the light blue wire going into the APPS & then checked the wire at the pcm again & .667vdc is a match. Did I do it right? Just making sure.

 

I followed those directions and it didn't do anything for me except threw some codes so I wound up putting back the way it was.

 

When I got the P0121 error code I just coughed up the $400 and bought a new APPS sensor and not worried about it...

As for the APPS adjustment I've never really tried it so I could tell you... But I've heard of several people saying it increased the idle speed...

 

I believe the Cummins electric circuits follow a common industrial norm where a sensor signal must be between about 0.5 volts and 4.5 volts to be a valid signal. The reason for this limitation is to provide for unknown voltage drops between different parts of the truck. As an example, the oil pressure sender must put out a voltage of 0.42 to 0.58 with a zero pressure (a Cummins spec.). You may wonder “Why not use 0 volts for 0 pressure?” but voltage drops in the wiring harness and connectors could corrupt the zero pressure reading and if 0 volts is sensed by the ECM it will trip a code for “Voltage-out-of-range”. The logic for this is to find a sensor signal that is shorted to ground. The same is true for the APPS. The ECM is looking for a small voltage with the idle validation switch closed. In other words if it does not see about 0.6 volts it will set a code. In my opinion this small APPS voltage is not at all critical as long as it is above about 0.5 volts. When the ECM is calibrated for the range of voltage from 0 to 100% of APPS travel all will be OK.

 

Here is my story. I replaced the APPS last summer after it went out completely. Then my brothers ecm went bad & I gave him my ecm & bought one from a fellow member for my truck. I installed the ecm & it started throwing PO121 code every once in a while. I checked the voltage yesterday & it was way off. This is why I did the adjustment. It definately changed something because it now idles/drives smoother. Time will tell if it fixed my issue or not.

 

J Lent- I'm pretty sure that's right. The voltage setting should be set for the sensor that is in there, so .667 should be correct

 

That was the problem. Code hasn't come back since I reset it. Before the reset, it would show up all of the time.