.I need someone who REALLY knows the Ford Mustang Engine?

p1131 means that the right side (passenger) sees too much air and is letting you know about it.... Most people are the slap a o2 sensor on it, but sometimes this can be caused by as simple as a vaccum hose unplugged.. Would take actually looking at it to determine the cause on the code... Some I've seen have been intake gasket leaks, some o2 sensors, and one K&N air filter that set this code...

The check fuel cap light is on.. IF riding around for a few days don't take it off, you have a major air leak in your gas vapor storage unit (aka Evaporative emissions..or that vapor you see when fueling your car sometimes)

It has to test the system for it to go off or a scan tool has to take it off for you... but cars usually won't pass inspection unless all the test the car runs itself have passed..so you are definitely looking at a shop to either fix it or spend enough money to pass it for another two years...

To learn more:

http://www.xcardiag.com/wholesale/new-ford-rotunda-dealer-ids-vcm-v77-jlr-v128-for-ford-mazda-jaguar-and-land-rover-12.html

Did you disconnect your battery for a few minutes. This will clear the fuel cap and other code as well. Basically what that code means is the your O2 sensor has detected a lean burning condition. Not sure if you know but the O2 sensor is located in your exhaust pipe. You have 2 before the catalytic converters (1 before each converter as you have 2 of them). You also have one O2 sensor after the converter. The O2 sensor detects how much fuel, air and emissions is going through the exhaust. Then the O2 sensor send a signal to the vehicles computer which tells the fuel injectors how long to stay open as the have a pulse and turn on and off delivering only a certain amout of fuel. Since it says it detected a lean condition, that could mean a number of things such as a bad O2 sensor, a bad fuel injector or simply a vacuum leak. Since the fuel cap light came on I would start by looking for a vacuum leak. Look around the engine for any old, rotten or loose vacuum lines. There are also a few "vapor" lines in the rear, under the vehicle around the gas tank. Check those as well. Also check you PVC valve. That's where I would start. I don't know it all and never will but have been a mechanic for over 25 years and happen to be a Ford specialist. In a nut shell the O2 sensor is not detecting enough gas and to much air which like I said is more then likely a vacuum leak since the other light is on. In most cases if it was a fuel injector not producing enough gas you would notice a performance problem like a miss or hesitation. After you check for leaks and correct anything disconnect your battery and drive it a few time and see if the code and light(s) come back on. If they don't then you have corrected the problem. If the check engine light comes back on you may be looking at a bad O2 sensor. Below is a picture of your exhaust and I have marked which O2 sensor is having the problem. Also below is a picture of what the O2 sensor looks like. It is screwed into the exhaust pipe. If you do decide to replace the O2 sensor DO NOT get a cheap universal type O2 sensor. Get a direct fit. Bosch makes a good one and they run about 50 bucks in my area. There are tests to perform in order to check the O2 sensor but will not put that on here at this point because I believe it's just a vacuum leak. If you have anymore questions or need any other testing procedure you can reply here or feel free to email me directly at [email protected] Hope I was able to help in some way. Here are your pictures.

P1131--Lack of HO2S-11 Switch

That code indicates a lean condition ,at the end of
a test,to correct for a rich condition

My suggestion --IF -- the front O2 Sensors have 100,000
miles or more,change them both

The code indicated one isn't switching fast enough
(front one ,right side or bank--11)

Need the Evap Codes, as the gas cap may not have
been the second problem, but there may be issues
with that system

Do not disconnect the battery or try & clear the codes
If you end up at a shop,they will need them

Your car is a metal object, so stay calm

I just spent 4 days diagnosing my wifes Lincoln LS
I resolved one issue ,only to have a bad battery,
put my professional scan tool software down.
You have to say calm & hang in there.