Doc

What does the O2 sensor after the Cat read? for example...air flow, heat, back pressure, etc...Thanks

Red

All OBD2 cars require a post-cat (downstream) O2 sensor so that it can meter catalytic efficiency. If your catalyst is missing, damaged or failing then the reported mixtures on the downsteam O2 sensor will very closely match what the upstream (pre-cat) O2 sensor reads.

If both O2 sensors are reporting similar results, the OBD2 system will throw an error code for Main Catalyst Efficiency. If you unplug the downstream O2 sensor, you will get two errors: O2S12 Low Voltage and Main Catalyst Efficiency.

No removing this sensor unless you fully replace the stock ECU. The Haltech has no "requirement" for an O2 sensor, but can accept input from two. Even so, I will be leaving both O2 sensors physically attached to the exhaust system so that I will pass the visual inspection part of the emissions checking here in New Mexico.

Doc

According to the Hyundai garage my after market exhaust and muffler (located after the second O2 sensor) caused the second O2 sensor not to read the proper back pressure, thus causing the check engine light to go off. The exhaust had been on the car for 9 months before causing a sensor to go off. After paying for the test, because Hyundai wouldn't cover it since the exhaust was modified, the check engine light wasn't on anymore. After 2 weeks of it being off it's on again.

It is true what they said about less back pressure will cause the check engine light to go on or just BS. I personally thought it was BS. This garage is known for ripping people off, but I had no choice at the time.

Decker

doc, I'm pretty sure thats BS. I checked on the hmaservice website the yesterday finding out the o2 sensore diagram. And there's nothing in there that'll measure pressure. It does have to warm up to work, but all it detects is the amount of oxygen in the gas. Thats it.

Red

As Decker explained, O2 sensors are physically incapable of measuring backpressure. An O2 sensor consists of a heater element and a sensor element. The heater element makes sure the sensor is at a certain temperature, and then the sensor itself takes the actual readings.

More than likely it's just the damned sensor going bad. I've replaced three O2 sensors in my car's life, and I'm already needing a fourth one. Sucky...

Sanz

Ok....I totally screwed up then. I put my first O2 sensor in the actual header and then the second one in the downpipe.
So basically I can just cover that one in the downpipe and then make a hole in the pipe after the cat. Does that make sense?

Shark Racer

Same here!!! I put on a Cat back system and K/N cone but nothing else. The car being 3 years old and having 40K mi on it still runs great, but the Check Engine Light has been off and on for over a year now. My service department says the computer is reading a lean condition. They did replace the O2 sensors both pre and aft but the light still comes on, more so in heavy traffic or climbing hills. I have seen some kind of device for Ford Mustangs that gets wired in line to the rear O2 sensor that will give the computer a false but correct reading. Anybody know if this could be adapted to the Hyundai???

http://www.mileliminators.com/

DESIGN

Shark Racer you may be getting a lean condition based on your air intake temperature. You live in the desert and with just a cone under the hood it could be extremely hot and therefore you could be running lean. BTW what is the elevation in Sin City?
What did you do with the air intake temperature sensor when you put on your cone. You may have either damaged it or you might have it just hanging there? Both of those things would cause your car to have false readings.

Shark Racer

The Air Temp Sensor is mounted in the back of the cone so it is reading air inside the cone. I was thinking on moving the cone down front to get some cooler air.