J Lent

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.

hamilton71801

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.

Mopar1973man

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...

bent valves

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.

J Lent

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.

Nick02Ram

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

J Lent

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