Code PO 171

If you have a check engine light, that means there's a trouble code stored. Get the code read and that should give you an indication of where to start

No i would go for the o2 sensors ,two are fitted one before the cat and one after ,use universal ones now the 4 wire ones are 40euros each but the 3 wire ones are 90 euros each, in the box is a wire conversion colour code chart but basicly the two wires the smae colour go together either way around and the other two look at the conversion chart

These old style check engine lights are pretty much just service reminders, and as such can be reset generally just by taking off the negative battery cable for about 1 hr. Still need to read codes, though. Check all your vacuum lines.

These codes mean that the oxygen sensor is reading too "Lean" on both the left and right cylinder banks and the engine control system can't compensate any further to solve the problem. You may have a dirty or failing Mass Airflow Sensor that is miscalcuating the amount of airflow the engine is taking in which in turn can cause both of these codes to set. If the sensor is reporting less air than the engine is actually taking in then the engine computer will add only the amount of fuel it should based on what it was told by the sensor which will cause both cylinder banks to run "lean". There could be something else going on such as having two bad oxygen sensors [very rare] but I do see these two codes together quite often and usually its the sensor that is causing it. Hope this helps a bit!

The system is too lean,this ,most of the time means that the fuel pump pressure is not enough,the fuel pump is going out,but change the fuel filter to see if it is the problem,if the fuel filter does not help,replace the fuel pump.

!70 and 171 are lean mixture codes, I'd start by changing the fuel filter. It's located under the truck, inside the frame rail, just about right under the drivers seat. The fuel lines are under pressure so you have to relieve the pressure before you disconnect the lines. You can let the vehicle sit for at least a 1/2 hour with the key off, or find the pressure test port on the fuel rail (on top of the engine) and bleed the pressure off there. You are going to need a special tool(s) to disconnect the lines. Helpfull hint push the line on to the filter while you insert the release tool(s), a little WD-40 or penetrating oil helps. Keep in mind your dealing with gasoline, keep your hot trouble light far away so there is no chance of gas splashing onto the hot bulb.

P1133 - Heated Oxygen Sensor (HO2S) Insufficient Switching Bank 1 Sensor 1
I would have the repair shop see if they either damaged or left disconnected the oxygen sensor for engine cylinder bank one upstream sensor, this sensor would have been disconnected if the oil pan was removed.

the line that feeds fuel filter is the 1 that connects to pump,other line is the return!

sounds like it could be plugged injectors or a timing problem.

i strongly believe it is your fuel pump