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My 2007 Daihatsu has failed the national car test on emissions. The readings at 3500rpm are:

Lambda: 1.10 (normal range 0.97 - 1.03)
CO: 0.02% volume (normal range below 0.20%)
HC: 11ppm (normal range below 200ppm)

To me, this sounds like the exhaust has a hole - which seems likely as it's rather rusted. I have yet to hold my hand over the exhaust, and listen for the noise of air escaping.

My questions are:

  • Can someone confirm that this looks I need to replace the exhaust?
  • Should I also replace the lambda sensor?
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  • Is it just me or do the CO and HC readings look to be in the stated "normal" range? – mac Dec 6 '13 at 17:32
  • The CO and HC are in the normal range, however for a 2005 car, these should be far higher. 11ppm, for example, is less than a new engine on this model of car! – Nick Dec 16 '13 at 11:19
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Do not put your bare hand over the tailpipe, it will get burnt. If you cannot hear an exhaust leak, puff puffing, you can pressurise the exhaust system with a rag over the tailpipe to make an exhaust leak more pronounced and tracible. The downside to this is that you can destroy the catalytic converter, so it is really not a good idea to do it. The way forward is to check the exhaust with eyes and ears. Eyes to see any black soot around a joint which means a leak, ears to hear a gas escape. The lambda reading you have provided may point to an exhaust leak, but without an exhaust leak it will mean a weak running condition of the engine. Prime suspects are the MAF sensor, MAP sensor, front oxygen sensor, fuel injector(s), or leaking or disconnected air hoses or vacuum pipes. Ideally with an ODBII scanner check the readings of the systems before you start spending money on replacement parts.

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  • You're right. I just realized that I got my lambda backwards (or rather confused it for phi). I've deleted my answer. Low CO should have been a clue... Embarrassing! – kahbou Nov 24 '13 at 18:30
  • No need for embaressment at all. At one time I worked for an organisation in the UK, Motability Finance, with a vehicle fleet of 480,000. Speaking to motor trade staff across the UK I began to wonder if the motor manufacturers were on a mission to make things difficult. – Allan Osborne Nov 24 '13 at 19:07
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Thanks for your advise, Allan. Got the car up on the lift, and was greeted with a nice large hole in the exhaust - large enough to fit two fingers into, and one of many. The botched weld job didn't help, either.

Hole in the exhaust

Botch weld

The exhaust was replaced with a stainless steel version (as opposed to mild steel), and a full engine flush was preformed - cleaning the injectors, pistons, fuel line, etc.

The emissions are now within limits:

Lambda: 1.01
CO: 0.03%
HC: 39ppm

Edit: I have to say, there is a noticeable increase in responsiveness, particularly at higher revs, with the new exhaust and after the flush. I wouldn't say that the car drives totally differently, but it's nice to see the change.

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