I am sure that most of us are familiar with this code by now. This time I have a Fiesta 02 Plate 1.4 16 valve that has recently had an ignition failure. I did warn the owner that he would probably have some comebacks. However my tests have raised some questions that I thought might be interesting.

Sure enough the car came back after a few days with a MIL light on & a Cat efficiency code. I did a four gas check & the CO was around 1.3 @ 2,500 RPM. This seemed a bit high to me. I wouldn’t expect that reading from a car with the cat removed. So I decided to scope the pre & post O2 sensors.

Sure enough, the downstream sensor was seen to be switching with similar readings to the front sensor, indicating that the cat was not doing a great deal. The problem that I have is that every now & then, both sensors will flat line & not switch unless an artificial rich or lean state is applied (carb cleaner spray or air leak)

When the sensors are switching & the scan-tool confirms a closed loop state, the STFT is seen hovering around zero & it responds as expected (it goes –12 ish when carb cleaner is sprayed into the throttle butterfly). As soon as it flat lines I notice that it has gone into open loop mode & the LTFT & STFT both stick at zero. It only goes into open loop whilst it is at cruising revs & as soon as it is allowed to idle, it will drop back into closed loop again & the sensors start to switch again.

I understand that closed loop cant be maintained under all condition e.i. hard acceleration, but I think it should be achievable at 2,500 RPM. Does anyone have any ideas about what else could cause this.

I have tried another O2 sensor & it reacted the same. I know that the Cat is knackered but I don’t want to fit a new one until I am happy that the system is ok.

Here is a capture……………