What is the difference between a upstream and downstream o2 sensor on 1998 Honda Civic DX. I would imagine one is on the headers and another mounted farther down the exhaust system, however if this is the reason for my check engine light do both have to be replaced? Thanks in advance for you assistance

 

One is before the cat, the other is after.

The exhaust gas temperature is measured by these sensors and if the rear one reads the same temperature as the front, the ECU thinks the catalytic converter is bad or missing. When your CEL comes on, it'll state which sensor is having a problem and you usually replace that sensor. If the CEL throws a code for both sensors and says something like "slow switching", then the sensors are likely to be good but you have an intake or exhaust leak.

 

What is your specific check engine code reading?

Just to clarify they don't measure temperature. In a nutshell the primary determines the amount of oxygen in your exhaust and determines if it is a rich or lean condition to adjust to that condition. The secondary is after the cat and is only used to determine the catalytic efficiency to determine if it is working or not.

 

Nice Geo

 

Don't know yet, were still scoping this car out. Were going to run the car up autozone and get the code checked out when we meet with the Owner again. Of course my in denial ADD father thinks to do that after we meet with the guy and test drive, but at least he thought of it.

 

sense made, granted the owner said he just replaced the muffler/exhaust system, the failed o2 sensor must be up front. Thats a lil scary. Do o2 sensors have a tendency to break or might there be an issue with A/F ratio. I know even throwing that out there is borderline stupid but I figured I'd ask just in case some vet has encountered funky upstream o2 sensors.

 

Usually the primary heater goes bad not so much the air to fuel is off. Although in these vehicles both 02s are in the exhausts manifold up front and the manifold often cracks causing these codes as well (P0135/P0141) which is going to mean hundreds of dollars due to the cat being built into the manifold. So look for a crack like this:


If its a p0420 code look at getting a new cat as well.