Emissions help

Had the jimny at mot and it failed on all the emissions high. Mot man said it was running rich and said could be the map sensor...I have diag equipment at work what do I look for when I diag it for the map sensor to see if it’s working? Is there anything else I should look for that could cause the high emissions?

Dont think any diagnostics will help you there. I guess you can measure ohm on the sensor, in the workshop manuals they often write how many ohm a working sensor should give.
But, it could just as well be the catalytic converter. I changed mine to a cheap ebay one and it was well inside the levels. Was the car driven and heated well? A cold engine will give off more, we usually let the car idle until hot and rev the engine to get it warm enough to pass the test

Temp gauge was showing up to temp when I collected it about 20-30 mins after he tested it so I guess it was the right temp. I’ve not got the money to be throwing a cat at it or anything for that matter can just about afford to get 2 wheel bearings for it! The tester said it was running rich so it gonna work on that theory for now and see if I can get it normal first

Running rich is often the pre-cat exhaust sensor gone out of spec.

It could be a really blocked air cleaner, the way to tell (apart from examine it!) is look at the fuel trim values, if they are right down it is not enough air, if they are turned up than it is the sensor.

A new generic one is only about £15, you have to get the one with the right number of wires, and the plug off yours is crimped onto the new one with the supplied metal tubes as you cannot solder to the wire.

Is that the sensor that is located in the cat itself? I’ve got the launch diag at work do you know of that could do the tests you need to do on it? Quite a long shot if anyone knows about it though

The one on the manifold before the catalyst.

How it works is the ecu just guesses the fuelling by looking in a table called the "map" (called running "open loop") when you accelerate based upon engine speed and manifold pressure, it doesn't really matter if it gets this wrong, as they aren't testing it, people fiddle with this predefined fueling when they "remap" the ecu. If it knows the sensor is kaput (usually if the heater goes open circuit), the eml comes on, and it carries on guessing what the fuelling should be. It may even pass the emissions test in this mode.


When you are running at constant speed & load the precat sensor is used. It is a switch that changes state when the fuel/air ratio is correct. The ecu goes into a "closed loop" mode, where it trims the fuel back slightly until that sensor changes state, then quickly tweaks it the other way a bit, so the fueling goes very slightly lean-very slightly rich-very slightly lean for ever. If you look at the live data stream, the sensor switches from below 0.3V to above 0.6V and back again around once a second.

If the air cleaner was blocked I'd expect the eml to come on as it can't go closed loop if it can't reduce the fuel enough.
Holes in the exhaust before the catalyst pull in air and affect the measurement too, but you'd know if you had one.


If the sensor seems to be working, but has gone out of spec., then the ecu does a great job of controlling the fuel to the wrong value and you fail the emissions test.

The sensor after the catalyst is just there to check that the catalyst has done its job and converted all the carbon monoxide into carbon dioxide.

That’s what I’m confused about there’s no engine light on. Should I try disconnecting the sensor to see if the car notices a fault and throws the light on? Is there anyway to fool it into fuelling differently without a remap?

The point that I'm trying to make is that if the sensor is giving the wrong reading, then the ecu merrily provides the wrong fuelling to match, all the time congratulating itself on getting it right, so it won't show any errors until the fuel trims max out before it can get the ok from the sensor.

I've had two cars fail emissions on running rich, both cured with a new generic 4 wire sensor, about £15 off eBay, neither put the eml on.

The one on the Jimny has been there at least 5 years now and just passed emissions this Saturday (0.01% CO, 5ppm HC, lambda 1.000)
I bet your lambda is below 1.

I'm not guaranteeing that a new sensor will cure it (as I'm at the other end of the interweb), but I think it will. For £15 why mess about?

The other possibility is a duff coolant sensor, so it thinks the engine is cold and is running open loop with a rich mixture anyway.

The live data will confirm what the coolant sensor says.

The readings on 1st test were
CO. Actual 0.497 max .200
Hc. Actual 86 max. 200
Lambda actual 1.323. Max. 1.030

2nd rest
CO actual0.515
Hc actual.70
Lambda actual 1.177

Natural idle
CO actual 0.296 max 300 does this make any difference to your thoughts now about the sensors?

facade wrote: The one on the manifold before the catalyst. If the air cleaner was blocked I'd expect the eml ......

I wish I had such a comprehension of how the ECU runs our daily rides like you do.
Back in the day if there was a risk of running rich we'd leave the air filter off but with all these dratted sensors I bet I couldn't do that these days.

It has been 30 years since I worked on fuel injection systems, and catalysts weren't in use, in fact we were only just moving from open loop systems, long lasting exhaust sensors were still new technology. (We were still working on the hot wire/film MAF sensors when I left)

Anyway, the high lambda indicates far too much air (lambda is [air/fuel]/14,7 so you are apparently running at 19:1 ) an exhaust leak anywhere would throw the lambda off as extra air will get in, manifold it will try and add fuel, post cat it will have good CO and HC readings..

The high HC indicate incomplete combustion, the HC is pretty much unburnt fuel. This could be a missfiring cylinder, caused by ignition fault, or over lean mixture (vacuum leak, possibly blocked injector or low fuel pressure), but I'd expect the CO to be low.

Normally high CO and high HC would indicate rich running, which is where we started, but that would make the lambda below 1.
I suppose rich enough to cause a missfire through plug fouling (clogged air cleaner, leaky injector, wrong information from sensors) would account for high CO and HC, but the high lambda seems wrong to me.

The only sensors are
water temp- open circuit here will run it permanently on choke giving high CO and HC but low lambda
Pre cat oxygen sensor- out of spec rich could give high CO and high HC but again low lambda, overlean would give high HC but low CO and a higher lambda.

Manifold Pressure sensor- a blocked pipe or failed sensor would completely throw the fuelling out causing rich or lean mixture.
Inlet air temperature (in airbox)- failure would affect mixture, as it uses air temperature, pressure and engine speed to work out the amount of air going in.
Cam & crank sensors - used to trigger the spark, measure engine speed and time the fuel injection, if one of these is intermittent it could cause a missfire, but the car wouldn't drive properly.


Fuel pressure affects the mixture, too low caused by filter blockage, pump failure or blocked regulator and it will run lean all the time, too high due to regulator stuck, or the vacuum pipe clogged and it runs rich.



As your mechanic suggested MAP sensor in the first place, have a look at it, I think it has a pipe with a small filter in, make sure that this is clear, and the hole into the inlet manifold.

Other than that, check for broke or split pipes, especially the pipe that feeds the vacuum hubs, and holes in the exhaust. I'd have a look for an exhaust leak around the manifold and down pipe, and check the EGR valve hasn't come off or something.



We've about reached the limit of what I can tell you now, Rhinoman knows a lot more specifics about Suzuki fuel injection than I do, I'd have just checked all the pipe work, and hoped a new pre-cat sensor would fix it

That may have helped a lot as the vac pipes aren’t connected and the vac hubs have just been ‘locked’ in place on the shafts(as if they’ll stay like that off road) but the vac pipes under bonnet are not connected so I will just double up the pipes so it’s a loop on the ‘t’ piece and see if that corrects things:)