Hi, I have a 99 mazda 323 saloon. The CO emmision on it is way over the limit on low revs. It's fine on high revs. I took it to the mechanic and he thought the problem was on the O2 sensor and replaced it. The poblem is still there. When we have a look at the O2 sensor output through an osciloscope we see that on high revs the signal fluctuates from lean to rich quite often. The mechanic says that this is the correct behavior. However when the revs are low and when the vehical is idle I see that the signal does not oscilate at all. It's stuck in lambda.

Has anyone seen this kind of problem before. Can someone please help me.

Thanks a lot.

 

Found out what the problem was. It was a faulty air mass flow meter. I replaced it and everything is OK after that.

 

O2 sensors do not switch at idle. 14.7:1 is the perfect air/fuel ratio so they set the PCM to achieve that without an O2 sensor input using TPS/ECT/MAF etc.

 

That's good to know for future reference. As I mentioned earlier the problem in my car was caused by a faulty air mass flow meter.

Thanks.

 

Yea i understand. I just wanted that out there because a good 75% of all o2 sensors replaced are still good. For some reason people think it is a cure all. I replace very few o2 sensors and when i do it is usually for the preheater failing.

 

Yes mine was replaced needlessly. He even suggested that something might be wrong with the catalytic converter or the computer of the car. If you are not careful you could end up spending a lot of money with these emissions issues.

 

HI I WAS WONDERING ???? MY 1985 RX7 FAILED THE SMOG , IT SHOWS 0 ON THE OXYGEN READING, THEY SUGGESTED THAT IT MIGHT BE THE DIVERTER VALVE??" AFTER THE SMOG PUMP" DOES ANYONE KNOW WHERE TO GET THIS OR HOW MUCH IT MIGHT BE? P.S. IT'S A MANUAL TRANSMISSION AND I CANT SEEM TO FIND THE OXYGEN SENSOR? DO YOU KNOW IF IT EVEN HAS ONE?

 

Most common cause of emission failure on those is the air control valve on the passenger side of the intake. Fuel injected models have a mixture adjust on the passenger fender. Little black tube looking thing which is sealed. Dig the crap out of it and you will see a screw. Be careful with this screw. You can only turn the screw 1/4 turn total. If you force it past that it shorts and can fry the ECU. It makes a HUGE difference in emission output. The best way to adjust it is with the car hooked up to and emissions tester. Just tweak it a bit and watch the readings til it is within spec.