From Newsgroup: uk.d-i-y

Yellow warning light came on yesterday. I used the diagnostic scanner
and found the fault was P0420 'Catalyst System Efficiency Below
Threshold (Bank 1)'. I cancelled the warning to see whether it was
intermittent or continuous. It has not come on again today. I replaced
the cat about two or three years ago. The vehicle it a Nissan Micra
2008 so I cannot justify replacing the cat again. I am thinking about
catalytic converter cleaning fluid to start. Any ideas? Could it be a
sensor fault?
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From Newsgroup: uk.d-i-y

On Thu, 30 Apr 2026 19:09:40 +0100
Scott <newsgroups@gefion.myzen.co.uk> wrote:

Yellow warning light came on yesterday. I used the diagnostic scanner
and found the fault was P0420 'Catalyst System Efficiency Below
Threshold (Bank 1)'. I cancelled the warning to see whether it was intermittent or continuous. It has not come on again today. I replaced
the cat about two or three years ago. The vehicle it a Nissan Micra
2008 so I cannot justify replacing the cat again. I am thinking about catalytic converter cleaning fluid to start. Any ideas?

Could it be a sensor fault?

That's probably most likely. A few years ago, I sold a 17-year-old
Fiesta, which I had owned for 15 of those years, and never had any
problems with the converter, but an oxygen sensor failed during that
time.
--
Joe

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From Newsgroup: uk.d-i-y

Scott wrote:

Any ideas? Could it be a
sensor fault?

yes, could be lambda sensor.
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From Newsgroup: uk.d-i-y

On 30/04/2026 19:41, Joe wrote:

On Thu, 30 Apr 2026 19:09:40 +0100
Scott <newsgroups@gefion.myzen.co.uk> wrote:

Yellow warning light came on yesterday. I used the diagnostic scanner
and found the fault was P0420 'Catalyst System Efficiency Below
Threshold (Bank 1)'. I cancelled the warning to see whether it was
intermittent or continuous. It has not come on again today. I replaced
the cat about two or three years ago. The vehicle it a Nissan Micra
2008 so I cannot justify replacing the cat again. I am thinking about
catalytic converter cleaning fluid to start. Any ideas?

Could it be a sensor fault?

That's probably most likely. A few years ago, I sold a 17-year-old
Fiesta, which I had owned for 15 of those years, and never had any
problems with the converter, but an oxygen sensor failed during that
time.

My 18 year old Fiesta has had a failed lambda sensor heater I think for
a few years. I thought it would cause an MoT failure, but garage said it
has two so it doesn't matter. Slightly annoying the engine fault light
stays on all the time.
--
Andrew
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From Newsgroup: uk.d-i-y

On 02/05/2026 00:12, Andrew Gabriel wrote:

On 30/04/2026 19:41, Joe wrote:

On Thu, 30 Apr 2026 19:09:40 +0100
Scott <newsgroups@gefion.myzen.co.uk> wrote:

Yellow warning light came on yesterday. I used the diagnostic scanner
and found the fault was P0420 'Catalyst System Efficiency Below
Threshold (Bank 1)'. I cancelled the warning to see whether it was
intermittent or continuous. It has not come on again today. I replaced
the cat about two or three years ago. The vehicle it a Nissan Micra
2008 so I cannot justify replacing the cat again. I am thinking about
catalytic converter cleaning fluid to start. Any ideas?

Could it be a sensor fault?

That's probably most likely. A few years ago, I sold a 17-year-old
Fiesta, which I had owned for 15 of those years, and never had any
problems with the converter, but an oxygen sensor failed during that
time.

My 18 year old Fiesta has had a failed lambda sensor heater I think for
a few years. I thought it would cause an MoT failure, but garage said it
has two so it doesn't matter. Slightly annoying the engine fault light
stays on all the time.

My Focus recently failed the MOT because the engine management light was
on all the time - was I misled ?


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From Newsgroup: uk.d-i-y

On 02/05/2026 09:31, Abandoned Trolley wrote:

On 02/05/2026 00:12, Andrew Gabriel wrote:

On 30/04/2026 19:41, Joe wrote:

On Thu, 30 Apr 2026 19:09:40 +0100
Scott <newsgroups@gefion.myzen.co.uk> wrote:

Yellow warning light came on yesterday. I used the diagnostic scanner
and found the fault was P0420 'Catalyst System Efficiency Below
Threshold (Bank 1)'. I cancelled the warning to see whether it was
intermittent or continuous. It has not come on again today. I replaced >>>> the cat about two or three years ago. The vehicle it a Nissan Micra
2008 so I cannot justify replacing the cat again. I am thinking about
catalytic converter cleaning fluid to start. Any ideas?

Could it be a sensor fault?

That's probably most likely. A few years ago, I sold a 17-year-old
Fiesta, which I had owned for 15 of those years, and never had any
problems with the converter, but an oxygen sensor failed during that
time.

My 18 year old Fiesta has had a failed lambda sensor heater I think for a >> few years. I thought it would cause an MoT failure, but garage said it
has two so it doesn't matter. Slightly annoying the engine fault light
stays on all the time.

My Focus recently failed the MOT because the engine management light was on all the time - was I misled ?

https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles/8-nuisance#section-8-2-1-1

8.2.1.2. Gaseous emissions


Engine malfunction indicator lamp (engine management light or rCyEMLrCO)

Turn on the ignition and check that the engine malfunction indicator lamp (MIL) illuminates and then goes off. On some vehicles it will be necessary
to start the engine before the MIL goes off.

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From Newsgroup: uk.d-i-y

Abandoned Trolley wrote:

My Focus recently failed the MOT because the engine management light was
on all the time - was I misled ?

Certain types of dashboard light must come on, to prove the light works,
then go off to prove there is no fault ... EML is now classed as a major fault.

<https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles/8-nuisance#:~:text=%28h%29,MIL>

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From Newsgroup: uk.d-i-y

On 2 May 2026 at 09:48:33 BST, Andy Burns wrote:

Abandoned Trolley wrote:

My Focus recently failed the MOT because the engine management light was
on all the time - was I misled ?

Certain types of dashboard light must come on, to prove the light works,
then go off to prove there is no fault ... EML is now classed as a major fault.

<https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles/8-nuisance#:~:text=%28h%29,MIL>

My Peugeot Partner (2015) has just started displaying a warning light - exclamation mark in a yellow triangle. Luckily it had the MOT last week - but even so, there's no way of knowing what it actually refers to without a code reader.

Can anyone recommend a reader so I can at least extract the code? A quick search lists basic readers from -u5 but I'm not sure what paying extra brings . . .
--
Cheers, Rob
Sheffield, UK
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From Newsgroup: uk.d-i-y

RJH wrote:

My Peugeot Partner (2015) has just started displaying a warning light - exclamation mark in a yellow triangle. Luckily it had the MOT last week - but even so, there's no way of knowing what it actually refers to without a code reader.

Can anyone recommend a reader

The el-cheapo generic readers are typically named ELM327, but I hear
quite a few complaints about them. I have a VCDS from when I owned Audi
cars, I'm sure it'll still do basic code reading from a Toyota but
probably not much more.
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From Newsgroup: uk.d-i-y

On 02/05/2026 10:20, RJH wrote:

Can anyone recommend a reader so I can at least extract the code? A quick search lists basic readers from -u5 but I'm not sure what paying extra brings .

Bluetooth readers are very cheap and an android app to read them is
free. As is generally a trip to your favourite repair shop 'can you read
and clear my codes please?'
--
"An intellectual is a person knowledgeable in one field who speaks out
only in others...rCY

Tom Wolfe

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From Newsgroup: uk.d-i-y

Andy Burns <usenet@andyburns.uk> wrote:

RJH wrote:

My Peugeot Partner (2015) has just started displaying a warning light - exclamation mark in a yellow triangle. Luckily it had the MOT last week - but
even so, there's no way of knowing what it actually refers to without a code
reader.

Can anyone recommend a reader

The el-cheapo generic readers are typically named ELM327, but I hear
quite a few complaints about them. I have a VCDS from when I owned Audi cars, I'm sure it'll still do basic code reading from a Toyota but
probably not much more.

The original ELM327 was a preprogrammed PIC microcontroller sold by an Australian company called Elm Electronics. One time they forgot to program
the protection fuse in the PIC with the result that the code could be read
out, and so the Chinese folks developed an ecosystem of clones. As usual
with those, they cut a lot of corners. Obligatory Big Clive:

https://www.youtube.com/watch?v=HrL6fDFt-Rs

So if you fish in the pool of clones of clones of clones, you get what you might expect. But other models do their own implementation the ELM327
commands and so talk to software that is designed to talk to the original ELM327. All the 'scanner' mobile apps are designed to do that - either
using Bluetooth (requires pairing), Bluetooth Low Energy (no pairing needed)
or Wifi (ELM commands go down a TCP socket to 192.168.0.10 port 35000).

I have an OBDLink SX (USB cable version) which doesn't talk to the apps, but
I set up a Pi Zero running OpenWRT and ser2net which turns the USB serial connection into a wifi connection that the apps can connect to. The OBDLink adapters (they have wifi and Bluetooth now, but didn't when I bought my SX)
are good quality but quite expensive. They run a lot faster than most clone ELMs which is needed if you want to interact with the CAN (logging, etc)
rather than just reading basic fault codes.

The scanning apps all have their own lists of recommendations, eg: https://www.carscanner.info/choosing-obdii-adapter/ https://wiki.torque-bhp.com/view/Bluetooth_Adapters https://priusapp.com/obd.html

Toyota's Techstream doesn't speak the ELM327 protocol, it uses a Windows
driver called MiniVCI and a cable speaking J2534. You can buy cracked
versions of Techstream that come with drivers for their own 'Mini VCI
cable'. Techstream and mini-VCI are a Windows thing, so you need a PC to
run them (I have a little Windows 10 tablet from back when they were cheap)

I can't speak for VCDS but I wouldn't be surprised if that also has its own cable rather than using the ELM327s. At one point I did look up the Hyundai/Kia tool and it was that way.

Generally speaking, in order of preference I'd probably prefer to get the
tool for your brand of car if such is available, but they are often awkward
(eg need a subscription). If that's not feasible then the mobile apps and
a good quality ELM327-alike are probably the way forward. I'd only try the dealer tools on the 'wrong' make if you have nothing better.

Recently I ordered this one as it was the cheapest on Amazon and it was
cheaper than ordering just the connector: https://www.amazon.co.uk/dp/B0DJ77CHY6

It works surprisingly ok for something that cheap, but it's Bluetooth LE
which a number of apps don't support - of the ones I tried I think only 'Car Scanner' does. (Turns out the connector was more integrated with the PCB
than I expected, so robbing it for the connector wasn't going to be that
easy). I wouldn't really recommend it, but at that price it's surprising it works at all.

(Also Bluetooth LE has no pairing process or other access control, so you
don't want to use that for long in case somebody else connects to it and
messes with your car)

Theo
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