Ongoing o2 probs

[gmdx]

Make Mazda
Model 6
Engine size/Type 2.0TS Petrol
Year 2006
Mileage 100000

Looking for some suggestions for an ongoing prob. MPG dropped to abt 75% and at the same time I started getting a rough idle and rough drive at low RPM. Gunning the accelerator would clear this, and acceleration has been unaffected.

I dont have the original Engine Management errors but they were all linked to o2 sensors (stuck) plus possible CAT, MAF. Errors were cleared once by mate and came back within 100 miles, and 2nd time after first garage visit.

Work already done (first garage) include MAF, sparks, air filter replaced, throttle body cleaned. No difference.

I then got a 2nd garage to replace the rear o2 sensor and immediately after this, the car never missed a beat for 2 tankfulls, but the fuel efficiency was still poor, and exhaust stank (rich?).

Finally, I got a front sensor and garage 2 replaced this last week. Immediate improvement to fuel efficiency, and no more Rich smell from exhaust, but the sluggish hesitation has returned, with one stall. No ECU errors (yet) but I'm stumped at what to look at next.

I didnt think the rear sensor controlled fuelling but replacing it did make an instant improvement to the idle/slow speed.

I have an OBDii reader and Torque passes the o2 sensor check since I replaced the from sensor, although I cant work out the tolerance levels and it takes quite a while to complete the test from startup (10 mins plus).

<HELP>

 

[Mursal]

O2 sensor will only operate when the engine is hot.

Would the richness be down to a coolant temperature sensor fault?

Double check the connections to all sensors (especially O2) for corrosion/green gunk

 

[Belle427]

Did you install oe spec sensors as sometimes pattern parts can be worse then the ones you took out.

 

[Avocet]

As far as I'm aware, the Oxygen sensor after the cat is only there to monitor the effectiveness of the cat. I don't think they play any part in the running of the engine - just put a warning light on if the cat isn't doing it's job. The one ahead of the cat is the one that the engine management system uses to adjust the fueling. The high fuel consumption suggests that the engine was running very rich. (which is why it would have gone well at full throttle). Rich running can be caused by (among other things) a failed coolant temperature sensor (as Mursal suggests) but shouldn't have improved by fitting a new oxygen sensor. The fact that it did so (for a while) suggests the sensor did, indeed, have something to do with it. I've had quite a lot of trouble with cheap, pattern and "universal" sensors and now only ever buy OE or Bosch (or other recognised brands like NGK). These guys have a good, helpful website and their prices are reasonable:

http://www.lambdasensor.com/

Well worth reading some of the technical blurb they have.