Definitely a bad sign that the car I'm borrowing while working on my Polo has begun to be troublesome.

So I'd done a short service on the Yaris 1.3 vvti 2nz-fe - except the spark plugs which are iridium jobbies and look great and the air filter. All fine.

Finally replaced the air filter 3 weeks later, then drove 20 miles including a fairly bumpy road and the exhaust split where pipe joins rear box. Surely coincidence. Okay, drove home 20 miles with box still mostly attached to the pipe (low revs a bit rough) - ordered a Klarius spare rear box, but could not get the old pipe off (clamp rusted) and in trying fully detached box from the pipe.

Drove under 2 miles with a very noisy exhaust from cold and the engine light came on.

Drove back home, engine cut out when warm and at idle unless throttle blipped or held at >700rpm.

I assume that the rear lambda sensor* was having a fit because reduced back pressure

would then dilute exhaust with fresh air. When started from cold it would rev itself rapidly to 2000 and back like an impatient driver at the lights (or a feedback loop is wide open). Once warm idle will drop below 500rpm - I hate that bargraph dash display - and engine cuts.

Now fitted the replacement box with grotty old clamp and it still has fluctuating revs when cold and will still cut out at idle when warm with the headlights on. There is a small exhaust gas leak at the reused clamp. So I've now got a new O-clamp and some paste. I suppose that'll increase the idle.

I'm fairly sure there's no air leak at the air filter.

How much learning has the ECU done about the free-flowing exhaust?

Will erasing the engine fault code fix the problem?

I suppose I'd better find out what ECU thinks is up?

(tried using Google but the "yaris cutting out" top search link went to some dodgy malware site).

* [ or modern bollocks as Yeti calls it]

Common fault this, have you tried setting fire to it? It generally improves most Yarises.

Joking aside, give it a code read, clear them and see what happens next, the thing with modern bollocks is if you don't, whatever you do is just guessing games.

If I were doing guessing games though, being a Yaris, I'd probably have a couple of quid on a coil pack.

Heater works quite well - so don't need to set fire to it.

Aircon is bust though - had it recharged once and it worked for a day then wheezed like an old man climbing stairs, but that was last summer.

I'll see what the code reader says.

Coilpack? Of course I might have disturbed ignition cables or MAP sensor plug when changing the air filter.

It starts easily, though.

It's coil-on-plug.

Fault code was 'bank1 too lean'.

Think bank1 is lambda nearer engine, and bank2 is post cat?

So I'll check the air filter housing.

There's a thin pipe that comes off the airbox, and it had become dislodged causing an airleak.

Erased the code, put the pipe back on its spout. All fine.

And now fixed the exhaust leak with assembly paste and new clamp.

Right, back to the Polo.

The Yaris gets 86bhp and 91lb.ft torque from 1299cc. I've got to beat that.