Does a f-100 have a straight 6 have an O2 sensor

bank one is left side top of engine standing looking at it , O2 sensor is located on exhaust manifold there is one on each manifold (right-left)

O2 sensors are always after the cat converter and HO2S sensors are always before the cat converter.
p0135 refers to heated oxygen sensor(HO2S) 1 bank 1-heater control--circuit malfunction
If it is a straight 6 cylinder engine with 2 cat converters it will be the sensor before the cat converter that connects to the manifold from pistons 1.2.3
If it is a V6 it will be the sensor that is in front of the cat converter that connects to the rear bank pistons 1.2.3. Use the special socket when removing/replacing the sensors to prevent wiring damage

If you do not know where O2 Sensors are ,I suggest
you let a repair shop test the ones you have,before you
throw parts at it.

No OBD Codes tell you to replace anything

Look on the exhaust pipe before or in the converter
inlet, for the upstream sensor & in the converter
outlet, for the down stream or monitor sensor

Sounds like an aftermarket O2 sensor was replaced. Should not be the problem unless your check engine light is on and giving that code. Also have you looked into your erg valve?

All About Lambda Sensors The exhaust gas oxygen sensor (EGO or O2), or lambda sensor, is the key sensor in the engine fuel control feedback loop. The computer uses the O2 sensor’s input to balance the fuel mixture, leaning the mixture when the sensor reads rich and enriching the mixture when the sensor reads lean.
I pulled this off a web site for you, I didn't know what it was either till I saw your thing here. Hope this helps.

Don't know if this will answer it. O2 sensors send info to the computer to make adjustments to the emissions system. Over time the O2 sensors fail which cause the computer to light the "Check engine" light. Originally the "Check engine" light was dedicated to an emission system fault. Now manufactures use it for a variety of abnormalities. To know what the "check engine" light means you have to either take it to a mechanic or buy a OBDII scanner (around $100) and read the code yourself. I like to read my own codes. These scanners are good for all modern cars. Instructions are included EZ to follow.
Long story short, don't worry about it the check engine light will come on when O2 sensor fails. Happy motoring!

DTC - 13 Oxygen sensor circuit open or no activity
DTC - 33 MAP sensor circuit error (signal high indicating low vacuum)
DTC - 54 Fuel pump circuit low voltage

Start with checking the wire leads to the O2 sensors and MAP sensor. Look for any damages to the wire harness and replace any parts damaged or faulty.

The O2 and MAP sensor take reading and sends it to the ECM to recalibrate the fuel injector pulse to adjust the amount of fuel entering the cylinders. When there is an issue to the O2 sensor or MAP sensor and even both, the ECM will be set a a factory default to run rich to keep the engine running and at times the engine may not even run. Normal fuel to air ratio is 14.7:1 - 14.7 air to 1 parts fuel and at default it could be as high as 12.5:1. Good luck and keep me posted, be glad to help you get your truck running 100% again soon and please rate. replacing the thermostat and temperature sensor would also be a good thing since the 4.3 runs best at 190".

This car has a transversely mounted straight six I believe, which is really unusual, but that means it only has one bank, so "bank 1, sensor 1" will be the upstream O2 sensor (between the engine and the cat) and "bank1, sensor 2" will be the downstream sensor (behind the cat).

O2 sensors are tricky. sometimes like anything else they fail. many times though the exhaust mixture they encounter is out of their range and that shows up as a failure.since you seem to have no other problems , I would go ahead and change them, but don't be disappointed if the code shows up again (at least you know it's not the sensors.) I don't like using the process of elimination when doing repairs but sometimes you must.

I have a 54 bel air customer brought in for me to install engine, but I do not know the firing order or the mark on distributor cap, could you please let me know what they are please, thank you very much. It is a straight 6.