I have a 2002 f150 king ranch 4x4. The check engine light came on about 2 weeks ago, so I took it to the auto parts store and they read the code which turned out to me my o2 sensors were dirty. I do not doubt this is the problem, because I drive it through the mud often. However, the engine light is now blinking at me. Is it blinking because I haven't taken care of the o2 sensors yet? Or did another problem come up? Also, I recently found a hole in my muffle which is getting fixed tonight, could that be the issue?

 

Just because the code reads O2 sensors does not mean that they are dirty. Remember, guys working at autoparts stores often are college/high school kids that are not mechanics.

It's not that common for an O2 code to actually indicate an O2 issue. Usually it's because of another issue.

Please, go straight to the store again and write down the codes. When you get them, post back up.

FLASHING CEL is not good!

 

In the hope that this will help you and others who need information on the MIL:
The MIL "malfunction indicator lamp" is required by regulations from the EPA and CARB (California Air Resources Board). The people from those agencies at the labs in Ann Arbor and El Monte dictate what and how different EMISSION CONTROL systems must be monitored. The purpose is to trigger a lamp that stimulates the driver to get the system fixed to keep exhaust emissions as low as designed. A faster repair should be less emissions.
The MIL "check engine" or "service engine soon" etc. will illuminate when a system doesn't react as expected. Hundreds of components and systems are monitored and the number and complexity of monitoring keeps going up. Again, systems that might adversely affect emissions are monitored.
When the MIL is lit it means something needs repair and/or checking. The procedure then is to plug a reader into the vehicle and read the fault code or codes, fix the problem, then erase the code or codes. You can erase the code without a fix but the code will soon return. An OBD II code reader is used to read and erase a code. OBD II means "on board diagnostics" generation 2. The codes are typically generic but not always, meaning the code 171 for a Ford is not necessarily the same fault as for a BMW.
Driving in mud and/or keeping your vehicle in top shape are irrelevant to the MIL monitoring. When a system or component does not react as designed the MIL is lit per the regulations. Each auto maker must submit thousands of pages of documents for every engine family each year for approval by the EPA and CARB and the systems are reviewed. At that time the regulators decide if the MIL system monitoring is sufficient or if more complex monitoring is needed. This means that one code may have a common fix and many other less common fixes. Could a leaking exhaust trigger the MIL? Depends on how the leak affects the sensors.
Bottom line: when the MIL is lit, get the codes read and the vehicle fixed.

 

Guy at Advanced claimed I had a bad speed sensor,I went to another store where they let me use the code reader an I was running lean in both banks,which could be because my fuel line has a leak that was found today. Moral of the story,don't rely on some parts store. Flashing CEL usually means it's misfiring,most common issue is gonna be coils. I fried one after going through several feet of water,CEL flashed and It felt like the truck was going to explode

 

Thank you! Brought it up to my buddy who's a mechanic today and he said exactly what you said. Still have to wait until Monday at least to get it fixed.