I have a 2000 Dodge 2500 diesel truck. The apps sensor comes on and off periodically. Code reads dtc po121. When on, truck hesitates, runs at 5 mph. I turn it off, wait for awhile then apps sensor light on dash goes out and truck runs fine. This is very infrequent. Question 1. does service light on engine dash with same code mean that the apps sensor is bad? Or can it be related to other fuel related troubles?

 

Either the sensor is failing, or there is a wiring problem. Sensor isn't too outrageously expensive though. Replacing it would be a good first step.

 

A qualified mechanic replaced the sensor. Truck ran fine for a week and then experienced same exact symptoms. Light came on, truck hesitates, stalls and then operates at 5mph. Mechanic then said I needed a new ECM, which is no longer made for2000 Dodge. They insinuated that the truck was not repairable as it had 300,000 miles and after market ECM's are problematic. I found another mechanic who fixed the truck. It had a fuel deliver issue and need a new lift pump and fuel filter. This mechanic said the apps sensor the first mechanic replaced had nothing to do with the problem, nor did it need a new ECM. I am trying to get my money back for the $710.00 part they charged me to replace, in fact the service manager even admitted the sensor was not the issue, the app sensor and they refuse to discuss with me. Frustrating. Just wanted to know if that code could be several issues and instead of a sensor going bad as the code read the same every time the truck failed? Truck runs fine now that the lift pump and fuel filter have been repaired.

 

I have seen bizarre issues that pop codes that are totally unrelated to the actual failure, but, those are pretty rare. Generally, the only way these types of failures get fixed, is by someone that has seen it before, or, sheer luck.....