What would a lazy Oxygen sensor cause

[lakemaligne]

I drive a 1996 2Ltr 16v petrol LX with 163,000 miles on the clock,and have just had the ECU replaced with a reconditioned unit to remedy an intermittent idle/stalling issue which I have been trying to fix for 2 years. Before the chap did the swop,he connected his "Network 500" diagnostic equipment and said that all the readings were well within limit and the engine was running very well.However he said that the "Oxygen Sensor" was reacting very slowly ,and said it was lazy and really needed replacing.He also attached his multimeter and said that the signal coming from the oxygen sensor was just outside the recommended parameters. With the new ECU in place the engine started and ran fine ,but the engine fault light was on and his test gear was still showing that the Oxygen sensor was reacting sluggishly and that this could be the cause of my idle/stalling issue at around tickover rpm. He cleared that fault light and since then the engine appears to run fine but the fault light appears once or twice on journeys.

The oxygen sensor was replaced about two years ago in an attempt to cure my fault.

My question is ,what would a lazy Oxygen sensor cause guys.Do you agree with his comments.

[steve_earwig]

I know if they're dead the mixture goes up and down like a yoyo as the engine tries to correct the reading - you can test this by trying to hold the engine revs about idle and seeing if they stay steady, if they go up and down by a noticeable amount then the sensor is probably goosed. Not sure about lazy, slow to react? possibly the same thing but slower

[no2lurch]

when you say oxygen sensor, do you mean lambda sensor? when i bought my 2.2, the seller said the lambda sensor needed replacing, and the revs would rise and fall slightly at idle like yours. he had already replaced what he thought was the sensor a few months before but still had the same problem. unknown to him there were 2 sensors. one before the cat and one after. he replaced the one before, and i repalced the one after. problem gone now. its an easy job, the hardest art is losing 75 quid for the sensor, unless you go for the cheap universal type but thats not recomended according to forums on the net. check yours has two.

[lakemaligne]

When the car is behaving itself,the tickover is rock steady at 950rpm. When she decides to play up,the tickover races up and down,but once on the road ,drives well,until i come to a halt and she stalls instantly.Once started again,she ticks over fine.

By lazy the guy mean slow to react.

Yes i mean Lambda sensor. My car is one of the early ones and only has one oxygen sensor which is fitted up-stream of the CAT.

I have noticed that my car will sometimes hesitate while on the move , like a flat spot ,almost like a momentary power loss,only for a second. Could this be cuased by a intermittent fuel pump or pressure rugulator of some kind ?