PO171 code

My check engine light has become the bane of my existence for months now. I was getting the 420 code but replaced enough stuff that finally satisfied that but then it was the 0120 code and replaced the fuel pump. It ran fine for a day and then the 0171 code came one for which I replaced the mas sensor and the light went away - for two days. Now it is back like a cockroach you can't kill. I read the thread from 2015 but those both said to do what I have done. Any other ideas? All suggestions appreciated even bad ones.
Thank you, Mike

 

Your truck is trying to tell you something, and throwing parts at it would only work if you get lucky.

P0420 - Catalyst System Efficiency Below Threshold (Bank 1):
1) Your o2 sensors need to be replaced (with OEM or Denso sensors), and depending on your year, and emissions standard (CA or non CA spec) you need the right sensors.
2) You have an exhaust leak.
3) Your catalytic converters are toast.

P0120 - Throttle Position Sensor A Circuit:
1) Test and replace the TPS with an OEM sensor.

Not sure why you replaced the fuel pump because of a 120.

P0171 - System Too Lean (Bank 1)
1) Too numerous of things to list.

Not sure why you would change the Mass Airflow Sensor (MAF Sensor) because of a 171, but I do see that as a common "fix". I imagine that's more a catch all. Did you use a Denso or OEM MAF?

You didn't mention what engine you have.

Ultimately- your ECU is getting bad data- and it's mostly to do with how much air is going in and what it's expecting to see on the way out (which effects the fuel trims on the way in) if it thinks it's running rich, it might try to lower the fuel trim or vise versa, resulting in the too lean code.

Let's start with some simple things. When is the last time you've replaced or inspected (you could have done some of these, but you didn't mention them specifically):
Spark Plugs
Wires
Coil Packs
Vacuum lines
Injectors
Fuel Filter
Air Filter
PCV Valve

A smoke test would go a long way in eliminating the vacuum lines and exhaust system for leaks. You want to make sure the metered air in and out of the engine isn't being altered (a vacuum leak would introduce additional air, and an exhaust leak would affect the o2 a/r sensor readings aka efficiency).

Hopefully you can see how the complete system needs to be in harmony for things to work properly, and why something upstream would affect results downstream.

What brand parts did you use when you replaced the sensors? The ECU doesn't play nicely with part store sensors. When you replaced sensors, did you do anything to verify the part you were replacing was actually bad?

 

Andy, Thanks, that's a lot of good input. Mike