I have an issue at MOT test where I get high CO readings. I've triple checked the following:

  • Exhaust leaks
  • New downpipe
  • Air/Vacuum leaks
  • Compression
  • MULTEC electronic Carb new idling control stepper
  • Injector test
  • Plugs/leads/coil/distributor/rotor arm all brand new OEM parts
  • Check/cleaned breather system

I'm now going down the route of diagnostics with a borrowed opel scanner. The vehicle is a UK Vauxhall Cavalier or Opel Vectra A in European markets. We are dealing with OBD1 here as the car is 94 model.

What I'm seeing with the 02 sensor, and the resultant air/fuel ratio is a pulse but the pulse looks a bit jaggy in parts, and also the cycle doesn't seem equal. All the tests were done with the vehicle stationary.

02 sensor 15 mins after operating temp reached

Air/Fule ratio

Note the AF ratio is taken at a different time frame than the 02 sensor.

Can anyone with experience in reading these sensors tell if this look normal or not?

Edit - attach print out data at operating temp.

enter image description here




I've now changed 02 sensor and things seem a lot better driveability wise and also looking at the traces for 02 and AF ratio. As shown below in before and after shots:

Before 02 Sensor Change

After 02 sensor change

AF Ratio before 02 sensor change

AF Ratio After 02 sensor change

  • Welcome to the site. What is the make, model and year of the car? You mention a carb, which would be rare on a car with an OBD system. Most cars with an OBD have fuel injection, do you mean an idle air control valve? What have you done with it, you say checked, does that mean you looked at it, replaced it, breathed on it, or what? – GdD Aug 18 '20 at 7:34
  • I already mentioned the make model and year, can you read it again? It's a MULTEC single point injection that sits in a unit, forget I said carb. If you are referring to the breather system i.e checked, if you read it again I said cleaned it, i.e. cleaned out the muck in it, do you need a more comprehensive explanation or photos? – Andrew Aug 18 '20 at 9:48
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    I'm not sure how I missed that, sorry @Andrew. I've gotten more coffee into my system, hopefully this will help. You didn't say what you did with the idle control, you just said you checked it. How many miles is on it? Also, have the O2 sensors been replaced? – GdD Aug 18 '20 at 9:53
  • @GdD No issues GdD, I replaced the idling control stepper motor if that's what you mean. I used to have issue where the revs would sit up about 2k when coasting to a junction or lights, changed the stepper motor, fixed the issue. There is about 120k mls on it, engine is well-maintained, and the top end was rebuilt about 5 years ago. There is only one 02 sensor on this car. I did change 02 sensor about 5 years ago but it wasn't OE/Bosch as far as I remember, so I'm thinking it would be good starting point to see it makes any difference, especially now that I can see the traces from it. – Andrew Aug 18 '20 at 12:11
  • Sometimes excessive ring blow by can push it over the CO limit on a test, I use to put a plug in the pcv hose to get them to pass, kinda cheating but beats rebuilding the motor to make the govt gods happy. – Moab Aug 18 '20 at 12:29

It's very hard to tell if the sensor figures are correct. O2 sensors become 'lazy' over time, with slower responses to changes in oxygen levels, this could explain the lagginess of the readings. O2 sensors can also become less accurate with age, and high CO is one possible result of that. So the sensors could be reading correctly part of the time, or they could be off, there's no way to know for sure. The high CO reading is an indication that something is wrong, and the O2 sensor is certainly one of the prime suspects for that, but there are other possibilities.

High CO is caused by a rich condition, which could also be caused by restricted airflow, faulty engine temperature sensor, faulty MAF/MAP sensor, or a problem in the throttle position sensor. If you know your O2 sensors are old it makes sense to replace them (presuming you have a post cat sensor as well, otherwise it's just the one), this may clear up your issue, or at least improve it. You could also look at engine temperature readings, air flow readings, presuming they are available, and see if they make sense.

  • There is a data for the map sensor voltage and pressure data, although have nothing to compare it to , the temp sensor was changed not too long ago. Only one 02 sensor in downpipe. I'd of thought any issues would have raised a DTC, but there doesn't appear to be any. I do wonder if it would only raise a code if the sensor wasn't cycling, or as open loop, but not when it was just slow/lazy but still working. I've attached a screenshot to original post that shows print out data at operating temp. – Andrew Aug 18 '20 at 12:28
  • It's hard to say @Andrew. There is an OBD2 code for an O2 sensor that isn't responding to changes, I can't remember what that is, but I suspect on an older OBD1 ECU there isn't going to any such thing, it's only going to detect a broken sensor. You are getting readings, and you say your sensors aren't that old, so they may not be the issue. Have you thought about running a can of seafoam through it? – GdD Aug 18 '20 at 15:13
  • I think if I can find at least a Bosch sensor then it would be worth changing to see if it makes any difference. I think I changed the sensor last time for a similar issue, I really can't remember if it fixed it but I don't think it ever did. Although before it was failing ON lambda and I fixed all the leaks in the exhaust and replaced the cat with one spare I had, I think at the time I was getting High HC but this time it seems to be high HC at fast idle. – Andrew Aug 18 '20 at 20:08

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