I've just had an MOT done and although it just passed, the results are completely different from last time:

Fast Idle Test:
Dec 2013/Dec 2014
CO:0.021%/0.0%
HC:5ppm/12ppm
Lambda:1.004/1.029

Natural Idle Test:
CO:0.078%/0.234%

No mods have been done between the tests and both were done at the same test center. Front lambda was replaced with OEM one in April 2013. There is no MIL on but my OBD reader shows the long term bank 3 fuel trim is around -28%, which seems very low (I presume it is accurate).

I would be really grateful for any advice on how to investigate this further, or an explanation on why the readings are so different.

Car is:2003 WRX

Regards

Darren

 

Could be a fault with the cat or Lamba sensor.

I had a fiat punto some years back which failed mot, garage said could be cat. Had it replaced still same problem, had it replaced again still same. Ended up removing panel filter and it passed lol

 

Maf sensor mate

 

Thanks jaygsi & Elb1

I've done some logging and the lambda sensors appear to be working correctly, the rear one is giving a rich signal nearly all the time!

I've ordered a new Denso MAF which I will fit and then reset the ECU.

Thanks for your help.

Regards

Darren

 

That negative trim shows that the air's should be too rich, yet the lambda reading is only just inside the lean limit for the MOT, does the car have all its cats?

I suspect you are wasting your money on maf, a failing maf would not cause the hydro carbons to kick up, it could be simply that the car was not hot enough at test

 

Agree. I always suspected that the dodgie MOT guys if you see you are "not clever" will fail your car on emissions saying they can service it, and "repair" emissions for another £100.

It happened to me with my mercedes this september, dogie MOT place goes your emission test failed. (and I knew cats have to be hot to pass, mine was dead cold engine).

so i started arguing = they passed it because i made them rev the engine up and keep it revvd till hot & pass.


Catalytic converters need heat to break them particles down.

 

as above. probably cold cats.

 

Thanks

Does anyone know what might cause the long term fuel trim bank 3 to be around -28%?

 

I had a friends subaru with -25% long term fuel trim. Means it's trying to pull out 25% fuel from base table.

Did a back to back comparison with original MAF sensor then a new one. This solved the issue. No more long term fuel trim. Car ran fine.

 

If u had a mate with a Subaru it makes life a lot easier to do the comparison before u spend money.

 

You can't have a 25% long term fuel trim the max for that is plus/minus 15%, the short term fuel trim is plus/minus 25% in a stock setup, there is bank 1 and bank 2, an Impreza does not have a bank 3, there is a fuel trim value that can be logged thats designated as "3" but its not a relevant log line. Its great being able to log the numbers but you need to know what you are looking at before you make choices to spend money if you see what I mean.

 

The conclusion is that:

1. The cats need to be hotter to improve the emission results (even though they are OE)

2. Cheap scantools can provide irrelevant/incorrect information - so check with someone who knows (e.g. Bob)!

Regards

Darren