1FZ 80 failed on emissions

[Theshootist]

I'm having trouble getting my 80 through the MOT on emissions currently. In the past 2 years I've had a new exhaust fitted with a cat, its also on new exhaust manifolds. The EGR is still in place but I'm failing on nitrous emissions. I'm wondering if this is something to do with the replacement exhaust system and cat, but its passed fine with this exhaust twice in the past and I understand the cat is really there for CO emissions. Could it be running too rich? Unfortunately my garage is out of ideas because they have no means to run diagnostics on an OBD1 car.

 

[MODVRS]

I didn't think the UK MOT measured NOx

 

[Towpack]

High Nox emissions can be caused by excessively high combustion temperatures from running lean/incorrect AFR. If CO and HC emissions are OK then I doubt the CAT is at fault.

 

[Theshootist]

Thanks All, yes the MOT measures CO, HC and NOx.

Interesting that it could be that its running too hot, ai will look into that

 

[Shayne]

increase or lower boost maybe

 

[Towpack]

The 1FZ-FE (petrol) doesn't have a turbo AFAIK.
Exhaust emissions tests were tightened a bit last year which could explain why it passed in the past (eh?) and fails now.

Emissions test

 

[Ben Stratford]

Would the highest quality petrol have different emissions than the cheap petrol?

 

[frank rabbets]

Very few MOT regs are retrospective, thank goodness. If you have the old paperwork you could see a trend in your NOx levels. Perhaps the tester could look online at your old results if no papers.

 

[Shayne]

Not sure it still applies but in the early days of all this save the planet nonsense just making sure the engine was full up to heat before the test could turn a fail into a pass .

 

[MODVRS]

Thanks All, yes the MOT measures CO, HC and NOx.

Interesting that it could be that its running too hot, ai will look into that

I asked ChatGPT.

"As of June 2025, the UK MOT test does not include a direct measurement of nitrogen oxides (NOx) emissions. While the MOT assesses emissions such as carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM), NOx levels are not directly tested during the procedure. This is primarily because accurately measuring NOx emissions requires testing under engine load conditions, which are challenging to replicate in standard MOT testing environments."

Doesn't mean it's right of course but I've certainly never come across it and am inclined to believe it.

 

[Shayne]

So take it to a garage that doesn't advertise for a test , the good ones don't need a website because local word of mouth keeps them to busy as it is .

 

[Theshootist]

Thanks All, its currently on a tank of shell 99 or whatever its called.

Happy to be corrected on whether NOx is tested. I shall likely try another MOT place.

 

[Michael80]

I’ve had this with a petrol one, check the rubber air intake after the MAF sensor, they split underneath and make them run lean due to unmetered air.

 

[Theshootist]

Well I've now been to the garage and spoken to someone who knows more. He confirmed that it is being tested to the standards of when it was registered. Its not NOx but Hydro Carbons that its failing on, so seems to be running rich. I've left him with a link to this forum where the method of reading codes via the CEL is explained.

 

[Macanoise]

This happened to me just over a month ago, never in the 18 years I’ve owned the motor, it’s a 95 series 3.4 petrol,because it doesn’t get enough use and my garage is only 2 miles down hill off the mountain into the village, it failed the emission baddly , fixed other repairs and booked it in for a retest, drove it in a few minutes before the test, and I took the long route to the garage so it was well up to heat and burnt off any shit petrol, PASSED WITH PERFECT RESULTS.

 

[Theshootist]

This happened to me just over a month ago, never in the 18 years I’ve owned the motor, it’s a 95 series 3.4 petrol,because it doesn’t get enough use and my garage is only 2 miles down hill off the mountain into the village, it failed the emission baddly , fixed other repairs and booked it in for a retest, drove it in a few minutes before the test, and I took the long route to the garage so it was well up to heat and burnt off any shit petrol, PASSED WITH PERFECT RESULTS.

You'd think the engine would be burning the fuel air mix more completely when hot I agree.

 

[Macanoise]

You'd think the engine would be burning the fuel air mix more completely when hot I agree.

Yes definitely, that what happened to mine, try speaking to the garage, then again it you haven’t been using it a lot or it’s a short distance to the MOT and it just sits there doing nothing until the test, and it’s started and put on the test machine without reaching the working temperature it will get bad results. The other thing you can do is clean the throttle body very carefully as not to create other problems, use a proper cleaning fluid from your local auto store. The only other cause could be air being sucked into a leak exhaust and up setting the Lambda sensors , O2 sensors.

 

[Shayne]

Its legal to drive on the road without an mot so long as you are traveling to an mot appointment , says nothing about how far you can go

 

[Theshootist]

I now have the emissions results. Two different people at the garage telling me two different results which don't match what's printed on the page!

HC emissions are acrually well within range. Its coming out with very high CO results and a low Lambda reading.

It has a new exhaust system with a cat fitted, which is less than 2 years old.

 

[Shayne]

The same garage ?