Mot emissions failure

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: nc 2006
  2. I’m based near: Luton
  3. I’m looking for technical help or recommendations on: __emissions failure on mot

Looking for advice please, mot failed on emissions I’ve attached photos, I’m thinking of replacing pre cat O2 sensor as OBD2 is showing a sensor failure (but not which one). Car is not great on fuel and do get a bit of a smell in car once warmed up.
Any help/advice/jokes welcome

Thanks in advance

Simon

IMG_7308
IMG_73083024×4032 3.08 MB

IMG_7307
IMG_73073024×4032 3.28 MB

2

What’s the OBD2 error number?

3

That’s a bit of the problem there’s no error code at all just when I run a sensor check or emission test it says failed on two differing odb2 apps

4

get someone with a ODB scanner and with the engine running have a look at the voltage on the pre cat oxygen sensor and see if its changing mine just showed 0.9 volts and didn’t change i put a new one in and it started to rapid fluctuating from 0.1 to 0.9 and put it back on mot emissions and it passed! pretty doing originally the same as yours though never showed a DTC report till it got really bad then it showed a p0031 heater control circuit low( bank 1 sensor 1)
I bought a second hand one off eBay for £40 and its now running better no soot in exhaust tailpipe and sailed through emissions and I only have the one cat no manifold cat

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5

I wonder if the cold start system is still active.

Try running live data if you can and see what temp the engine is actually getting to.

My 2006 NC seemed to show ok temp on the gauge but was only running at about 65c

I changed the thermostat and the temp now gets to 80c but the dash gauge still shows the same.

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6

Thanks for reply I’ll check voltage out I’ve order one from eBay to try the same. Yours sounds very similar to my issue

7

Thanks for reply I did change the thermostat about 5 months ago for a differing issue the odb2 apps show temp around 85c constant

8

High HC and high CO most probably over fuelling. The 1st sensor controls fuelling so most probably this one.
But an OBD reader should be able to confirm a fault.

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9

HC isn’t very high so the mixture is firing ok, just too rich. Certainly sounds like the lambda sensor is not switching.

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10

The HC on this engine on a properly running cat should be in single or max low double figures. This one is tested at 190 and171 which is only just below the pass figure of 200 which is a very generous value for a modern engine in good condition.
For comparison on last MOT my 2006 2.0 had a CO of 0.002 and an HC of 1. The lambda is also very low which would indicate a sensor problem.

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11

Thanks to all for help and guidance after 2 failed secondhand o2 sensors from online, autolinkuk came up trumps with a new one at very good price. Sailed through the MOT today.

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