I have a 98 mustang 3,8 and i just got a P1131 code how do i fix this? does this need my immediate attention?

Did it blow the spark plug out of cylinder 3 ?
Code 303 is for a loss of power from cylinder 3, code 1131 is for a problem with the O2 sensor in bank one and code 1151 is for the same problem in bank two. Codes 1131 and 1151 could be caused by the problem source for code 303.

code p 0303 refers to cylinder 3--misfire ===causes among others is --engine mechanical fault
the bang could be a major destruction of the engine

Hope this helps you or future parties of intrest. P1131 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1 P1151 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2 I would get under your stang and gently clean outside portion of o2 sensors using a scotch brite. Also if you have so dielectric grease pull electrical connections to o2 sensors clean them and add grease. These trick eliminated the o2 sensor OBD1 codes on my 1987 Ford mustang gt

Fix the misfire concern on cylinders 5 and 6. That will probably take care of the lean codes p0171 and p0174. The catalytic converter is probably damaged from the misfire concern....that's what is setting the p0420 code. Take care of these concerns first before fixing p1131 code

P1131 - Insufficent Switching 02 Bank 1 Sensor - 1 Sensor / Lean.

Sensor 1 on bank 1 is the passenger's side upstream 02 sensor.

Start by cleaning the MAS and Filter and see if this helps. If not, you will probably need to replace the O2 sensor.

It COULD also be a bad No.2 injector I believe

think it could mean the mass air flow sensor in the air intake pipe ,but i would test this with a automotive meter and not a code reader to be honest

3 digits. follow this link for more info. Good Luck.
http://autorepair.about.com/library/ts/obd-i/bl-dtcs-98.htm

Injectors need cleaning, As being left for time its got dirty, Run a fuel system cleaner through the fuel, Or get an professional to clean injectors for you with a high pressure unit.

Code p1131 manufacturer control fuel air metering, on a 2003 Ford Taurus. Where is part? Is there an easy way to see if it can be fixed without buying the expensive part?

po301 thru po306 all misfire codes cylinder 1-6 remove plugs and replace them..... set the plug gap first
i suggest u replace the wires also if they never been replaced all these misfire codes can set the others and one is for the oxygen sensor ( 02s or HO2S) CIRCUT MALFUNCTION do u know how to check wireing systems
do the tune up plugs (set gap) .052-.056 change the fuel filter also. clear the codes hows it run
this info is for 98 E-250 4.2Litre
what engine do u have 4.2L or 4.6L i know it is a v-6 by the codes