This car runs lovely, no problems whatsoever, but failed MOT due to slightly high CO, HC and Lambda. It read Fast Idle 3.3% CO, 233ppm HC and 0.90 Lambda. Last year and year before it showed similar problems, and on second measuring it would suddenly pass with all green, like a switch was turned on. We then assumed cold engine.
Now engine was warm enough and the problem would not go away.
I have no warning lights on my dash. It is model with a computer under passengers feet, so I cannot scan for any error codes.
All accessible engine compartment connectors were treated previously with contact fluid (Stabilant 22) and show no corrosion. There are no other contact or other problems.
I measured sensors with a digital multimeter:
MAF measured OK from 1 to 2 V.
IAT measured 30 kOhm at room temperature falling to 5 kOhm when heated, OK.
ECT measured 38 kOhm at room temperature falling to 3 kOhm in boiling water. This is different from Haynes or Chilton manual (2.5kOhm falling to 300Ohm), but I have another brand new sensor bought for this car and it measures the same as the one in the car, so I assume error in manuals., or change in sensors for 1998+ and not covered in manuals. Also when I disconnect this connector, the rad ventilator starts almost immediately (it probably reads this as overheating) when I connect back the ventilation stops.
O2S (front oxygen sensor) measured partly OK, the voltage being close to 0V when engine cold, and between 0.15V to 0.85V when hot, behaving as described in manuals, meaning oscillating. It steadied at 0.85V with connector off. The resistance of the heater element (this is 4 wire unit) cannot be measured, I believe this is dead. The sensor wires look good, not damaged or burned, flexible with intact sleeving. I couldn't get to rear sensor, if there is any.
There is 170,000 miles on the clock. The exhaust and catalyst are probably original, as there is no record of the change. There is a full service history until 3 years ago when I gave up on Mazda service centres, in disgust.
I would like to know what is the likely problem before taken to cleaners at Mazda service centre, as I tried all London ones over the last 10 years and was not happy.
Thanks for reading and contributing.
ayoshak
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check the 02 sensor feed wires, should have 12v and an earth from ecu, if you have that then you need to test the resistance through the heater circuit on the sensor, if its open cicuit ie no resistance then bin it, you can bench test the o2 sensor but if the heater circuit is open then there is no point, dont be tempted to buy a cheap replacement get a decent one preferably a bosch if they do one for your model (dont be tempted by the ones on the auction website, lots of very impressive poor quality copy's) if you do replace the o2 sensor dont expect an instantanious result as the ecu will need to recover/relearn, give it a run around mixed driving mainly in town then get it back on a emmision tester, check very carfully for airleaks split pipes etc as well'Regards TB
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