Catalytic converter

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.

It would be worth looking at the O2 sensor readings using the scan tool as you rev the engine. Presumably the ECU is seeing something on the downstream sensor that it doesn't like, but not bad enough to throw an O2 sensor fault (ie it's not open circuit or anything like that). . With the scan tool you can see whether the sensors are misbehaving or there is a fault with the cat which the sensor is picking up.

I'm sure there must be plenty of videos of what good and bad O2 readings look like.

Theo

I don't think mine is sophisticated enough. It really only shows the fault codes, of which there is only one (P0420). However, this gives me a starting point to speak to the garage.

yes, could be lambda sensor.

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 ?

Engine malfunction indicator lamp (engine management light or ‘EML’)

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.

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.

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 £5 but I'm not sure what paying extra brings . . .

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.

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?'

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:

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:

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:

(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