I recently purchased a 2001 jeep Cherokee.the check engine light will not come on. all other lights come up.cannot pass inspection until light comes on.also found out I needed 3 o2 sensors because of

SOURCE: Check Engine Light

It sounds like you may have a failing Mass Air Flow sensor, seems like Ive been seeing these tied together quite often. Its a little tricky but a good repair shop should be able to test this sensor. I dont want to tell you to replace it without having it tested cause they are a bit pricey but check around, you never know. Hope this give youu another idea where to look.

SOURCE: Check Engine Light - O2 Sensor - bank 1 running lean

you should have three oxygen sensors on your car, one for each bank of three cylinders; should be towards the Y pipe on each exhaust manifold. Additionally you should have one on the exhaust pipe before the cadilitic converter.

SOURCE: P0155 & P0135

O2 sensors are tricky. sometimes like anything else they fail. many times though the exhaust mixture they encounter is out of their range and that shows up as a failure.since you seem to have no other problems , I would go ahead and change them, but don't be disappointed if the code shows up again (at least you know it's not the sensors.) I don't like using the process of elimination when doing repairs but sometimes you must.

SOURCE: 2001 Honda - New Catalytics & O2 sensors -- new 0420/1739/1456

I would send the car for a smoke test for the emissions leak...Find out where the leak is comming from..Repair it..Reset the codes, then do a drive cycle and see what happens...

Kirk Augustin

• 2019 Answers
• Posted on Jul 28, 2010

SOURCE: 2006 Jeep Grand Cherokee 5.7L engine, Hemi, sensor code P1129

The OBD is not that simple.

All it is saying is that it does not like the air/fuel mixture.
That can be from fouled spark plugs, an open gas cap or other intake leak, a bad O2 sensor, PVC foam on the air mass sensor, throttle position sensor, etc.
It is just like any carb engine when the mixture is bad.
You can't just read a number and know what to replace.
You have to have a feel for the symptoms.
And if there are no other symptoms, it could be the code is simply the only thing wrong.

Personally my best guess would the air mass sensor.
Look the inside of the oil cap. If there is foam there, then there is likely foam in the air mass sensor as well. PVC systems always throw foam into the sensor, and the only solution is to splice longer hoses into the system and into a condenser of some sort. This is worse in northern climates, in the winter. Most cars in cold conditions have this happening all the time. But as long as the light goes off eventually, don't worry about it.