My Yaris is due for an MOT this month. The last MOT report says that CO is right on the limit, everything else is normal HC, Lambda etc., as shown below.

The CO has been creeping up every year, and the code P0172 has recently came up which means "System Too Rich".

The car is 19 years old and when it's first started I can smell petrol outside and sometimes when I get out after driving it I can smell it too. I've tried Cataclean and i've also cleaned the MAF.

I'm thinking it's the CAT? Any thoughts?


Emissions Report

  • Welcome to Motor Vehicle Maintenance & Repair! Have you changed the O2 sensors or still running the original ones? Mar 4, 2018 at 14:37
  • @Paulster2 Thanks! The pre-cat sensor is 2 years old and the post-cat sensor is 10 years old.
    – Ross
    Mar 4, 2018 at 14:41
  • The post-cat wouldn't have much to do with it, but the pre-cat would. Seeing as how it's only 2 years old, I wouldn't think it would be an issue. Have you checked for vacuum leaks? Are there any exhaust leaks? Have you cleaned your MAF? Mar 4, 2018 at 14:49
  • @Pᴀᴜʟsᴛᴇʀ2 - Yes, there doesn't seem to be any vacuum or exhaust leaks. I've also cleaned the MAF.
    – Ross
    Mar 4, 2018 at 14:50
  • Also a fuel pressure leak down test... Possibly a leaking injector?
    – SteveRacer
    Mar 4, 2018 at 15:12

1 Answer 1


Maybe a leaking injector

High CO is typically rich but your lambda says otherwise - I'm unfamiliar with your exact testing (England? - Petrol what is that? :) ) so I don't know what it means. I typical lambda sensor (oxygen sensor) averages about .5 volts and swings wildly at idle. They way you know its working is the swinging back and forth).

A too rich signal means the Lambda sensor is reading high too often and the computer is having trouble adjusting.

If it were EGR, I would expect all the cylinders to adjust via the lambda reading and you would more or less drive fine.

With one injector leaking, at or near idle you would have one cylinder running a bit rich, and the other adjusted lean to compensate. Under a moderate load (steady on highway) probably less likely to notice.

Another possible clue. Your startup smell. The gasoline drips down the cylinder and one cylinder ignites with extra fuel. You might notice it taking a bit longer to crank/start the car (needs time for fuel pressure to build in the fuel lines).

So... Shut off the car, measure fuel pressue. If it drops quickly, and with all the other symptoms, probably a bad injector (leaking). Probably the cheapest thing you can do is replace them (only 3 right?)

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