My car has a system too rich bank one code and multiple cylinder misfire. When I purchased it I changed the fuel pressure regulator right away and the oil pressure switch because each had a leak. Oil got into the spark plugs but I dried it up , replaced the plugs, boots , coil packs , housing unit , and had the ignition module tested and it was good. I also changed the fuel filter, and the o2 sensor off of the exhaust manifold. Advanced auto parts told me my timing was jumping from 7 to over 20 in neutral, and its misfireing pretty bad. What could it be ? What do I do next ? Thanks

codes p 0300 through refer to misfires on each cylinder with the end number ( 301 is no 1 cylinder , 302 is no 2 cylinder ans so on)=== causes --engine mechanical fault , ---wiring---ignition/ fuel ---injector/s---ECT/ MAF sensor/s ---ECM
most misfires are from injector faults like dirty/blocked tips, low fuel pressure , no pulse or problem pulse from ECM rather than plugs and wires and coils
codes p0171 refers to system to lean bank 1===causes ---intake exhaust leak---air system---maf/vaf sensors ---fuel pressure / pump---injectors ---heated oxygen sensor ( HO2S)
code p 0172 is system to rich-====causes ---intake blocked ---EVAP cannister purge valve ---fuel pressure --EGR system---injector/s ---HO2S
couple these two codes with the misfire codes , I suggest you look at the injectors, fuel pressure (too low) from EVAP problem not venting tank, fuel pressure regulator, EGR system

You either have a bad catalytic converter causing too much back pressure or you are running rich. I think cylinder 3 and 6 are on the same side of the engine (except opposite banks) right? Locate the fuel pressure regulator. Is it on the same side? I've seen those go bad and allow fuel to be sucked into the vacuum line causing a rich condition/ misfire on two or three cylinders on that side of the engine.

Could be a few things I can think of, leaking injector, leaking / bad fuel pressure regulator, or defective O2 sensor(s) making the computer read high air content and the ECM is dumping in more fuel to compensate. You can check your plugs for gas fouling to isolate either a single cylinder or multiple cylinders will give you an idea of what to look for. If only very fouled on a single cylinder, probably a bad or leaking injector on that cylinder. If evenly distributed among all cylinders, fuel pressure regulator. If you have a check engine light on, get the codes read at Autozone (free) and it may tell you if it is an O2 sensor problem. Random misfire or multiple misfire codes would bring me back to the fuel system components. Get this taken care of right away and change your oil when fixed, you can cause severe damage to the engine with fuel going excessively into the cylinder(s) and / or crankcase.

how many miles on the odometer, mines 95k.
ah a v6, P0306 is #6 misfiring. 2.5L V6
300 is all 6 misfiring.... oops.
it is. for sure.
misfire is cause by many things, only 3 in physics, but many real world. (compression + spark +fuel rates)
i works this easy, way.
i have a full scan tool not just a code reader (near useless it)
i make sure my V6 (I have an 04) holds closed loop at idle and 2500 rpm. i f not , fix that first.
easy, it is. just look, bingo, Open loop, (out of control)
fueling is out of control.
but, first i do that tune up. in the manual, 60k , 120k, 180k, see?
do that first. , spark plugs dont last foreever nor filters.
then i replace both front 02 sensors at 100k miles. there design life.
V6 has 2 front sensors. (rear can , just sit and be ignored)
if it misfires. now, i toss a coin.
compression or fuel pressure.
i can do both in 30min flat but i toss coin lands on fuel.
i check fuel pressure and its.
60psi, fuel regulator is bad,
20psi, same, or bad pump or bad 12vdc to pump, 8v to pump is bad wiring.
one thing that helps is to find out if the engine is running too rich or too lean.
and if there are cracks in the exhuast near the 02 sensors.
i force the EFI rich and lean watching both 02 sensors in my scan tool
to see if i can force them lean or rich.
on the v6, we can introduce air leaks, to the plenum.
this will cause lean, and the o2 most react to my lean forcing
or its dead.
i can force richer, if i suspect lean or 02 shows lean, by
removing the Fuel pump regulator hose (vac) and plugging hose fast.
this causes the FPreg to go to high pressure. max.
causing RICH>
i watch the 02 and see it go rich, if not the 02 is bad or you have exhaust leaks, or dead cylinder causing the 02 to fib.
you have 2 banks, and both banks are running off 1 MAF
so you can look at both 02 sensors (front) B1S1 and B1S2
and see if both react the same. to my forcing's.

i dont think you have 3 bad cops out 6. no.
if any cylinder misfires on bank one, then bank 1 o2 goes nuts
same with the Bank 2. this can aid troubleshooting having 2 banks.
the MAF can not lie for 1 cyclinder nor one bank
the 02s can lie for each bank,
both are key to diagnosis.
let me give an example say
injector 1 leaks.
causing 301s
v6
246
135 left
this is your lay out.
my guess is its not lost compression (no test guess, mostly useless)
not 2 bad cops one on each side will cause 300s
not one bad cop. as other side would be ok.
not one bad injector.
my guess, is fuel pressure is way off.
but///???
or maf running lean (dirty, spary clean it 12inches away not close)
low fuel pressure
too high fuel pressure.

all comments are for engine with 60k mile service done,
60k, 12ok, etc.

The cat coil, and engine module could be bad.

Check Engine Light Codes and Meanings
Long blinks Short blinks Problem
1 1 Camshaft position sensor
1 2 Mass air flow sensor
1 3 Engine coolant temperature sensor
1 4 Speed sensor
1 11 EVAP control system (flow)
1 14 Fuel injection system right bank (too rich)
1 15 Fuel injection system right bank (too lean)
2 1 Ignition circuit
2 3 Closed throttle position switch
2 5 Idle speed control
2 9 Fuel Injection system left bank (too rich)
2 10 Fuel Injection system left bank (too lean)
3 1 ECU
3 2 Exhaust gas recirculation system flow problem
3 3 Front left O2 sensor
3 4 Knock sensor
3 5 Exhaust gas recirculation temperature sensor
3 6 EGRC-BPT valve
3 7 Closed loop control
3 8 Closed loop control
4 1 Air intake temperature sensor
4 2 Fuel tank temperature sensor
4 3 Throttle position sensor
4 7 Crankshaft position sensor
5 3 Front right O2 sensor
5 4 Automatic transmission control unit
6 3 Cylinder #6 misfire
6 4 Cylinder #5 misfire
6 5 Cylinder #4 misfire
6 6 Cylinder #3 misfire
6 7 Cylinder #2 misfire
6 8 Cylinder #1 misfire
7 1 Multiple cylinder misfire
7 2 Three way catalytic converter (right side)
7 3 Three way catalytic converter (left side)
7 4 Evaporative emission control system pressure sensor
7 5 Evaporative emission control system
7 7 Rear O2 sensor
8 1 Vacuum cut valve bypass valve
8 2 Crank position sensor
8 3 Absolute pressure sensor
8 4 Automatic transmission control unit
8 6 Evaporative emission canister purge control solenoid valve
8 7 Evaporative emission Canister purge control solenoid valve circuit
9 1 Front Right O2 sensor
9 3 Evaporative emission Canister vent control valve
9 5 Crankshaft position sensor
9 8 Engine coolant temperature sensor
10 1 Front left O2 sensor
10 3 Park and Neutral position switch
10 5 EGRC solenoid valve
10 8 Evaporative emission canister purge volume control valve
11 1 Inhibitor switch
11 2 Revolution sensor
11 3 Improper shifting to first gear (Automatic transmission)
11 4 Improper shifting to second gear (Automatic transmission)
11 5 Improper shifting to third gear (Automatic transmission)
11 6 Improper shifting to fourth gear (Automatic transmission)
11 7 Solenoid valve lock up function
12 1 Shift solenoid B (Automatic transmission)
12 3 Overrun clutch solenoid valve (Automatic transmission)
12 4 Torque converter clutch solenoid valve (Automatic transmission)
12 5 Line pressure solenoid valve (Automatic transmission)
12 6 Throttle position sensor (Automatic transmission)
12 7 Engine speed signal
12 8 Fluid temperature sensor
13 2 MAP/BARO Switch Solenoid Valve
13 5 Fuel pump control module
13 8 Cooling fan

P1131 -1. Misfire on bank 1
2. Vacuum leak affecting bank 1
3. Fuel injector problem bank 1

P1130 - 1. Fuel pressure high or low
2. MAP sensor dirty/defective
3. Vacuum leak on engine
4. Leaking fuel injector or fuel pressure regulator

P0172 system too rich bank 1. P0300 random multiple misfire. All are a common symptom of either bad fuel pressure regulator or leaking injector. Need to pull off intake plenum (upper) You should visibly see a clean side or part of intake manifold (lack of carbon buildup) Reconnect fuel lines. Energize fuel pump (turn on key to run) Most likely you will see fuel dripping out of rear (fuel pressure regulator) or nylon lines to each cylinder (fuel injectors). Once you see which is leaking will need to replace. Once they are repaired will need to clean or replace spark plugs & change oil & filter.

P0171------ system too lean bank#1
Po174------ system too lean bank#2
For these 2 codes, try cleaning the maf sensor (mass air flow), that is mounted in the big black hose that goes from the air filter housing to the throttle plate, if it is dirty, that will cause those 02 sensors to send a signal to the computor to lean the air fuel mixture. If cleaning it doesn't work, the sensors will probably need changing.
P0300---- Random/multiple cylinder misfire
P0316---- Cylinder misfire at start up---- first 100 revolutions.
On those 2 codes, you have to do some testing to find out why there is a misfire.
Possible causes:
dirty/malfunctioning injectors
weak coil pack
fowling spark plugs/wires
Low fuel pressure

P1127 Long Term Fuel Trim mult.,Bank1 System too Rich

P1129 Long Term Fuel Trim mult.,Bank2 System too Rich

P1250 Fuel Level Too Low

P1300 Misfire detected Reason: Fuel level too low

P0154 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)

P0300 Random/Multiple Cylinder Misfire Detected

P0303 Cylinder 3 Misfire Detected

P0304 Cylinder 4 Misfire Detected

It sounds like there is a lot going on.

Good Luck

Brian