I have a 2001 Dodge Ram 3500 5.9 litre gas engine. I just replaced the map sensor because I was getting P0301 and P0308 codes after swapping out 2 o2 sensors, fuel injectors, rotor and distributor cap, and spark plug wires. I am now getting a P0121 code. Why is this happening?

Yeah, you could do that. Bank 1 is always the engine side where the number 1 cylinder is located. Sensor 1 is the first sensor in the exhaust system, nearest the engine. Sensor 2 is always located farther downstream in the exhaust.

P1297 DODGE Description The Engine Control Module (ECM) detected no change in MAP sensor voltage.

- MAP Sensor harness is open or shorted
- MAP Sensor circuit poor electrical connection
- Faulty MAP Sensor
- Faulty ECM

cam shaft position sensor, sparks plug,and cables

first pull them out and check for black carbon lines on the spark plugs if the check engine light is on have it scanned at a parts store the code should lead you to the cylinder example p0301 is cylinder 1 misfire p0302 is number 2 up to p0308 that will help anyway

the code is map sensor too high, too long. certain programmers with boost foolers can cause this. if running a programmer disconnect and if problem isnt fixed replace the map sensor.

P0455....Evaporative Emission Control System Leak Detected (gross leak)

P0138....O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

Distributor pick up coil has failed ( hal effect ) , remove cap and rotor and you will see it usually under a plastic shield . Just remove and replace .. they are fairly cheap.

it will be the o2 sensor after the cat.. check the connections. the first bank should be the right side of the engine while sitting in the car. look at that side of the exhaust.

If you need further help, reach me via phone at https://www.6ya.com/expert/ayron_57e1b351b7242eef

Fuel injector cleaner commonly has a low success rate, and will not help if it's more than one cylinder that is missfiring. The fact that you have a "rattle" noise, leads me to beleive the engine is not receiving fuel on one or more cylidners. If it is all the cylinders, than the fuel pump may be the culprit, or the fuel pump circuit. But if only one cylidner is missfiring than it is a possibly an injector. A code P0300 refers to all cylinder missfires, and any code between P0301-P0308 distinugiushes a particular cylinder.

Misfiring
1- a general electrical tune-up (spark plugs, wire set, cap & rotor) is a basic start to cure this issue. Wiggle the dist shaft while you are in there, if you have one.
2- Then a good carbon cleaning / throttle-body cleaning.
3- Vacuum leaks at any of the hoses that connect to the Intake manifold; especially the MAP sensor connection.
4- If that does not help then the O2 sensor could be bad or drifting.
5- Funky coil/coil-packs
6- Theoretically a microsecond glitch in either the cam or crank shaft sensors would cause this, but I am not aware of this as a typical cause.
7- Lastly a funky PCM but again it is not common for this to cause miss-firing.

Note for the V8 then also:
a) bad erg valve (this dilutes the combustion gases and hence raises the combustion temp
b) a leaky plenum/Intake manifold gasket. Leaking Intake gasket symptoms

Also can check the TSB 18-48-98 Ignition System Cross Fire/Secondary Ignition Wire Induction (Complete Version )

Hope help with this (remember rated this). Good luck and have a nice day.

dont know lots about this witout engine size but some have 1 coil per cylender possible coil is bad or plug 02 sensor will cause some problems but a miss fire will cause it to run like trash