Ok next question, can an upstream o2 sensor go bad but not throw a code? The reason I ask is because I scanned my jeep and the upstream seemed to be working correctly, it was going high low high low. The reason I ask is because my jeep has been asting up and seems to be running rich. I ran some seafome thru it and it seems to of smoothed it out a bit.

 

Well yes it can be bad and not throw a code,but when it goes bad chances are it will start then stall. now i don't now what year you have or should i say; obd 1 or obd 2. obd 2 it will stall. im not sure how it will run or not run with obd 2.

If you are obd 2 you have a different issue and not an 02 problem. it sounds like a tps sensor or possibly iac control valve.

if i am missisng or wrong about something others will chime in and correct me.

 

Its a late 1996 motor so im assuming obd 2

 

ok then i would check the 2 sensors i mentioned and keep adding the seafoam treatments, b/c it could be dirty injectors. if one or more of the injectors were bad, you would get a code. but poss dirty injectors.

 

in world of OBD, car needs to check warm up periods to see if o2 sensors are ok or not.it requires consequent failures are required to throw a code. lets say three wam up periods for a code. in general several o2 sensor related problem may occur. heater circuit may fail, or only its wirings may fail, or sensor can be cracked too. some of them may nto be showing codes due to external conditions.

i saw all problems on several cars which we own. once i owned a nissan. it had a broken post sensor and was not throwing a code since i forced car to rev a long time on highway to see if it would go into limp mode or not. than it decided to throw a code and started to stall when engine was warm.

when cold o2 sensors are out of order fyi.

 

can you just buy the same type of o2 sensor for the upstream and down stream?

 

it depends on manufacturer specs. majorly two sensors measure the o2 percentage in same way. you need to find specified model for your vehicle.
once i owned a small nissan SUV, i installed bosch oem it worked flawlessly. but on VW jetta, two of different bosch oem models failed after 10 miles driving.

if voltage readings after warm up period and after sensor is heated up same, or change in voltage reading are exactly same or close, ecu thinks cat is gone.

when you connect two voltmeters to output wires of sensor, and if there is no cat or broken cat, two multimeters acts exactly same. they show change in voltage at same time. if there is healty cat converter, second acts slowly than first one.