my 99 ford escort check engine light is on?

Question

all the scanner said was air to fuel ratio incorrect replaced mass air flow sensor had code erased but can back 4 days laters what else could be going bad

Answer 1

An incorrect air/fuel ratio can be caused by many things, if it is rich it can be caused by a fuel pressure regulator, bad injector, MAF sensor, O2 sensor, ect. If it is lean you could have a vacuum leak, cylinder misfire, bad fuel pump, clogged fuel filter, or again a bad O2 sensor. Many people will tell you a misfire will make it rich but it won't, the PCM is looking at the amount of oxygen in the exhaust, if a cylinder misses it blows all the intake air that wasn't used during combustion down the exhaust causing the O2 sensor to show lean and the PCM to add fuel. If your scanner has PID monitor functions if you watch the O2 sensor you should be able to tell if it is working or just flat lined. That will at least give you some direction to start looking looking in. Depending on what code you retrieved from the PCM your local library should have books that will give you a better description of what the code is actually telling you. It makes me laugh when I see Autozone comercials telling me they will retrieve codes for free, all they are doing is selling parts, I have been a mechanic for 26 years and I really wish fixing a car was as easy as pluging a computer on it and replacing a part.

Answer 2

oxygen sensors also adjust the mixture ratio, but should show a trouble code of their own. If the mixture was rich, make sure your air filter is clean, and the intake tube (from air filter) is not kinked or restricted. If the mixture is lean, check fuel filter, fuel pump pressure, and try having injectors cleaned.