451 smart fortwo

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Hello there.
Is anyone having problems with the lambda sensor (O2 sensor), I had a code P0139 and replaced the lower sensor 3 months ago and recently the warning light come on again with the same code, even when the sensor is new.

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I just had what I hope was a false alarm over the upstream lambda sensor.


Story goes like:

Tuesday it rained a lot. Car was parked at the office in the rain all day. Tuesday afternoon the car started very rich and nearly coughed it's lungs out. Turn it off. Wait a few seconds. Restart and all seemed well.

Wednesday afternoon the engine warning light came on. Scangauge indicated P0134 - O2 sensor fault.


Thursday morning the light and error code persisted. Drove to the local Mercedes dealership (who are not franchised for smart!) and enquired about sensor replacement and booked a service for next week. They phoned me back later... one is available out of Brisbane... cost over $1000. Eeek... forget that!

Did web research. Boschautoparts US list replacment sensors. Couldtn't find a listing for Bosch in Australia. Read all the O2 sensor blurbs - the modern ones are supposed to be good for 100,000 miles / 160,000 kms. My smart is 2008 vintage. It's not expected to have sensor problems for a few years yet, so after-market spares are hard to come by.

Thursday afternoon, engine warning and error code persisted. Set the Scangauge to monitor O2 and MAP - all seemed OK... mmmm... stopped at the supermarket. Hopped back in and the error had cleared. Has stayed clear since.

My smart is a pulse turbo. Not available in the US. I had to order the turbo option when buying new (and wait 6 months for delivery). Even in Australia, the turbo is scarce, so informed user experience is rare.

The smart O2 sensors are of the pre-heat variety. They need to reach an operating temperature (over 90C I believe) before they function properly. I have learned the hard way to wait at least 15 seconds on a dead cold start for the sensors to heat up before starting. If I don't: the car may not start or start with a very rich mixture. It seems that the O2 sensors used in the smart are a bit finicky. They also are probably of the wire intake vented type (dunno - can't find precise specs) - which means they may occasionally suck in moisture or fumes that will upset them. If upset, they can turn on the engine warning light and raise an error code.... Chances are it's a false alarm. Check the wiring connectors for contamination and run a diagnostic (such as monitoring with a Scangauge) before repalcing them if the car has done less than 100,000 miles. At least that's my current hypothesis.

Simple way to clear engine light warning is to disconnect the battery for a while (say an hour to be sure).. Then see if the codes re-appear. Browse the UK Evilution web site for sensor problems - they suggest that removing it and cleaning it in gasolene may all it needs. These things are not meant to fail in three months!


Hello Ian long time not hearing from you, very valuable comments, always.

Well I think I may have a false alarm too, first like yours it didn't start one day, it was raining too, it took me several minutes and tries to get it on, finally it started and I went away, later on the light come on, next day I took it to the Mercedes-Smart dealer, they diagnosed and told me it was the O2 sensor, they wanted $600 for the sensor and $400 to change it, of course I said no way, nevertheless they charge me $140 just for the scanning.

I started to look for the O2 sensor on the web, Europe was expensive 250 euros plus shipping, out of league of course, I did found in the US a Mercedes Benz dealer willing to get it for me for around $200, keep looking and found in saparkplugs.com PN 25186 for $91.

I ordered and have the doubt since the US model is a non-turbo and my is a turbo passion 2009, but when received it was the exact one, the manufacturer is NTK just like the original, the only difference was the length of the cable 50 mm vs 22 mm, not a big deal, a couple of turns and tape and it is done.

I took the courage and looking at the Evilution page, I changed myself in around 2 hours for disassembly the rear end to get access to it, then disconnected the battery 15 minutes and the code went away, no more problems for 3 months until recently, again it is rainy season, disconnected last a week, and back again.

Investigating the matter in web found that the code does not affect the engine or the fuel being dispensed, so there are no symptoms to the car being noticed, the fuel economy is the same I even check the emissions and they are within the specs, just the annoying light, I taped for not seen it, feel the car ok but, I know the light is there and worries me...

I have checked the old O2 sensor and did all test I have found and seems to be Ok, the car had only 27,000 KMTS when replaced the sensor, that is why I don't think the new one has failed with only 3,000 kmts.

On the weekend I will take a deep look, see if I can test the new one on the car and see if I can find something, I will reset the light and so on...I have been reading that may be the MAF sensor, I will take a look, checked the fuel cap just in case it has a leak, then will see.


Today I untaped the light started the car and it is gone, don't know when it went away but I will monitor it to see if it gets back.

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