2003 dakota 4.7, 5 speed. truck always got 18 mpg. Around 100k it dropped to around 16. recently had clutch and tranny replaced. "coincidentally" had many other things happen while at the shop. I now have these two codes and my truck is getting the best mileage it has ever at 19.5 for three tank average. what is happening? after i replace the sensors will my mileage go back to 18?

 

Sounds like the bad O2 sensor is making the PCM adjust the fuel mixture in your favor. Most likely will go back to the prior mileage whenever you change it. Others will chime in. Dont know if running lean all the time can hurt the engine or not.

 

Running lean will possibly cause damage to components in the long run, especially if you ever push it hard (towing or WOT).

It is weird that both sensors would quit at the same time. 131 is for the front sensor (in front of the catalytic converter), 038 is behind the cat. I am wondering if the shop didn't pinch the wires or leave them unplugged, or maybe even damage the sensors themselves, when they were replacing the transmission.

First, I'd take a peak under there and see if they simply left them unplugged by accident, or if the sensors are missing or damaged. If the latter proves true, I advise you go buy a new one for the front sensor, as that is the only one that truly matters. The front sensor(s) tell the computer how to trim the fuel, so that the air-to-fuel ratio is correct. The rear sensor(s) just tell the computer if the catalytic converter is fine or not. You could possibly ignore the rear sensor code.

Good brands of oxygen sensors:
NTK (NGK)
Denso

Brands to avoid (poor quality):
Standard
Bosch

 

if the upsteam sensor is bad could it cause the downstream to give a false reading?

 

I had an 01, and mine ran lean or rich, not sure which. My 02 sensors were going off bad, threw codes, and always misfired when I first started the truck, until I floored it and get it past the bogging, it was fine. Within 1000 miles, I got a bad rod knock. Not sure if it was just bad engine, but I took very good car of my truck. So I would watch out, just to be safe

 

Missed until I floor it.. I take very good care of my truck...

 

How long for the codes to clear?