P38A Faked mot exhaust sensor

pete12345 · 2025-06-06T08:02:03+0100

Here’s a few pics I took of readings before & after a 40miles drive I’ve got live data aswell will try to up load it later can anyone spot a major change or problems just from the pics

Mark Piercy · 2025-06-06T12:54:13+0100

If you can get the nano In graph mode (not sure). The short term fuel trims should average around 0 in the plus or minus.
The lambda sensors should have an undulating wave pattern indicating more or less oxygene pass-through the exhaust and cat.
If the short term fuel trims are running in the plus range and upwards there could be an air leak somewhere that is making it compensate for a lean mix due to un-monitored air changing the emissions and the system is trying to balance itself

kermit_rr · 2025-06-06T15:40:53+0100

Mark Piercy said:
If you can get the nano In graph mode (not sure)

i had to save the data to SD card then open in excel to do that, but maybe i just found it easier to see

pete12345 · 2025-06-06T16:02:18+0100

U lost me mark short term fuel trims what does that mean in none mechanic language cheers

Mark Piercy · 2025-06-06T17:39:45+0100

Aha, OK. So the engine in a fashion adds fuel and subtracts a bit of fuel to allow more air to go into the cats to boost the efficiency of the cats to burn the emissions off. This is why the fuel trims lift and lower and in turn the lambda sensor or exhaust sensors do a similar thing when watched on a graph.

pwood999 · 2025-06-07T10:55:06+0100

If it's a Thor motor, the O2 sensors should flip between 0V to 1V. If it's not a Thor, then you are using the wrong Nanocom menus !!

Long term adaptations should adjust over several drive cycles to get the fueling in the right ballpark. Short term adaptations adjust over shorter timescales to adjust the fueling while driving day-day. If either are way out the engine will be far too rich or lean, hence O2 sensors will not change voltage.

pwood999 · 2025-06-07T12:00:08+0100

Also you didn't state your actual issue in the titel or first post ??

Has it failed MOT on emissions, or you have CAT's with no internals & want to adjust the fuelling to pass the MOT ? P38 sensors are Pre-CAT so only tell the ECU what's coming out the engine. The CAT's are there to clean it up a bit more.

In either case the O2 readings tell you if the engine is too lean or rich. Then you need to find the cause. Usually air-leaks, MAF, throttle body, fuel-rail pressure, etc.

pete12345 · 2025-06-07T12:01:47+0100

Well thanks for that chaps now my mind is really blown what I’ve done is taken readings on the nanocom now I’ve replaced the old air filter and cleaned the maf so will take reading’s again this evening and look for any changes then I’ve found another page on the web with some readings so will compare mine with them but what I’ve already checked I think there nearly all correct numbers mabey on lot avg fuel trims might be out a bit

pete12345 · 2025-06-07T12:06:18+0100

All it says on check is exhaust lambada readingafter 2nd fast idle outside limits

pete12345 · 2025-06-07T12:07:57+0100

As always thanks for expert advice

marjon · 2025-06-07T12:27:36+0100

pete12345 said:
All it says on check is exhaust lambada readingafter 2nd fast idle outside limits

What I find strange is they "bypass" engine speed and oil temp and dont actually give "actual" numbers?
Is that the way they do it in the UK now?
So you havent got a clue what problem you are trying to find. Or even if it was in the correct range when readings were taken.

If they dont know those figures how can it be a correct reading?

J

kermit_rr · 2025-06-07T12:37:05+0100

Hmm could be an exhaust leak

kermit_rr · 2025-06-07T12:39:28+0100

Lambda is the air fuel ratio.. a high number means it's too lean, but if the HC and CO is ok then you could well have air being sucked in the exhaust via a small hole

pwood999 · 2025-06-07T13:00:01+0100

After the engine warms up, do a recording to the SD-Card for the ECU Inputs screen. Then upload the CSV file here so we can look at the live data.

Pug957 · 2025-06-07T16:55:18+0100

Lambda sensors can fail at anything over 75k miles so if they're older than this it'll be worth replacing them regardless. Look at nline catalogues for Bosch and NGK for their part numbers and search those and find a decent price.

Only saying that because none of the trims etc I'm seeing would have me worrying about how the engine is running. Check engine lights are normally triggered about 20% deviation and 5% is normally considered normal conditions which vary due to temperature, altitude, humidity etc.

The only thing the engine has to know what the mixtures are like are the pre-cat sensors. They tend to fail lean so not great for your engine.

In your case I'd be doing pre cat sensors and looking for holes/leaks after the o2 sensors which are letting air in.

P38A Faked mot exhaust sensor

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