My Mx5 is an early '98 Mk2 with only 36K miles and it’s in VGC. As far as I can see, it has a single lambda sensor, on the manifold. This was changed last summer, less than 5k miles ago due to my impression that it was slightly sluggish and the car now starts first time, hot or cold and runs very well, with no misfires. However, it has just failed the MOT test with high CO levels.
No lambda readout appeared on the results sheet. I’d not seen this before so I quizzed the tester/operator about it and he said their machine won’t give a figure if it was out of the tolerance band. The garage had fully warmed the engine (I was in attendance) and I’ve no reason to doubt their competence. They know I work on my cars and so there is no benefit to them if the car was incorrectly tested, which I’m certain it wasn’t.
I’ve ordered another new sensor, which I’ll fit asap. However, if this doesn’t do the trick, I obviously need to look deeper and I’m trying to get the car fixed within the ten working days allowed.
I suspect another culprit could be the coolant temperature sensor. What I don’t know is whether or not the early NB coolant sensor has an output to the ECU, in addition to the coolant temperature gauge on the dash. I can see that it’s a three wire sensor and suspect it must do so. However, looking at Mr. Haynes’ wiring diagram, it’s not shown. That manual only mentions where the sensor is and how to change it, which I’m fully able to do. Before I shell out for an unnecessary part, can anyone give a definitive answer?
If this isn’t the problem, I’m probably looking at the MAF or cat. convertor next.
It’s just an answer about the coolant sensor at this stage.
There are two temperature sensors on the car. The front one a thermo switch only switches on and off the cooling fan.
The one behind the engine near the bellhousing talks to the ECU and is a temperature sensor advising the cooling system temperature and in turn gives in car entertainment information to the temperature gauge. It is not in the least a true temperature gauge.
This can go faulty but it tends to give intermittent rough running as it tells the ECU to richen the mixture at normal running temperatures rather than when the engine is cold.
If you want to check if it is giving problems get an LED diagnostic kit from Autolink UK.
In my experience probably a few years ago the coil packs have been changed due to a fault and that damaged the cat with unburnt fuel at the cat, that can take a year or two to fail an MOT as the cat can take that time to loose its functionality.
I would go for a replacement cat and if you want a new Lamda sensor.
I do have an LED type diagnostic tester. It shows no faults.
I’m aware of the location of the sensors. If the one behind the left hand cam cover defintely feeds the ECU, I’ll test it. Any info on the correct impedances would be useful.
I’m in the same situation as you with a MOT failure due to a CO reading.
I changed the O2 sensor, cleaned the EGR.
Got Cataclean in the fuel.
Tomorrow I’m going to clean the MAF and the throttle body.
I’m also going to see if the MAF is adjustable.
The MOT tester wanted to see what would happen if we took the electrical connector off the MAF hence switching the ECU in to a default mode.
But we couldn’t get the connector off (it was dark). I’ll look into it tomorrow.
The Coolant Temp sensor is on the list but I think everything is pointing at the CAT failing.
If the car starts and runs ok,hot and cold ,with no starting issues,then it is unlikely to be coolant temp sensor.Is there any way you can get the emission fail sheet readout?You are entitled to it as part of the mot test.
Yes, thanks, I’ve got the emissions readout. The CO is above limits. The HC is well within. The car is running rich. It became even richer as the engine got hotter.
I understand that everyone will have their own ideas about what the fault could be, based on their own experiences. I’m trying to work through everything in a logical order and carry out what checks I can easily do at no cost, with my multimeter, before I start changing the more expensive parts, such as the cat.
At this stage, I need to confirm it’s not the coolant sensor. If the coolant sensor is not working correctly, it may continue to signal a cold engine irrespective of engine temperature, which will result in the ECU unnecessarily keeping the mixture rich.
Cleaned the MAF and throttle body by spraying carb cleaner in the intake.
Took it for a blast and then for a retest.
The mechanic put the probe up the exhaust and low and behold it was all at passed levels with CO at 0.13%. BUT by the time the tester put all the details in to do the actual retest it had climbed back up to 0.6%.
Drove home to get my carb cleaner and went back. By spraying carb cleaner into the intake the CO levels dropped to 0.1 to 0.2% BUT the lambda levels rose to a fail.
In the end we gave up and I’m going down a CAT replacement route.
Then just pulled out of the garage and went down the road and the car just cut out and would not start. Called out the AA who confirmed there was no spark at the plugs.
The Engine fuse under the dashboard had blown. Why it blew he could not tell me. He did give me some new fuses in case it happened again.
Little badger, thanks for the Utube link. I think the sensor being tested in the video is only a two wire one, the one I need to check has three wires so might have a dual output, which is really what I need to know. My new lambda sensor arrived today so I fitted that. I’ve also changed the spark plugs because although the colour of the old ones looked normal, the gaps were a bit too wide and a change can’t do any harm. A quick road test made me think there was a bit more response, but it could be imagined. I was otherwise involved today so no time for anything else. I’ll test the coolant sensor tomorrow in situ, with a cold and then warm engine. It won’t prove much without knowing exactly what the readings should be but it appears to be working I might attempt another emissions test as things stand. If it fails again, I have a used cat available.
Shame about this; the MOT tester commented that the car’s structure was almost like new underneath and that he hadn’t seen many so good at this age (coming up to its 19th birthday at the end of this month). It sailed through the test apart from the emissions problem.
I’m not sure why you’re testing the voltage at the sensor. It’s the resistance/impedance of the sensor itself that determines whether it’s working properly or not.
