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Hi guys, my 12 year old, 143k miles GTI caught me out this morning, it has literally started on the button every day for the last 7 years. This morning it was like trying to start some 40 year old thing with carbs on an icy morning

I get home from work tonight and plugged in, this is what I got which cleared and hasn't come back:-

16804/P0420/001056 - Catalyst System; Bank 1: Efficiency Below Threshold

Also on the readiness page I'm getting 'Failed or Incomplete' on the following
Oxy sensors
Evaporative emissions
Cat converter

Any ideas about the bank 1 thing? Is this the injector?

Sorry for the long post!

Hi bud

That fault is for the Lamda sensor pre cat ie the front closest to the engine. Its located in the manifold to the right hand side

Check the wiring on the front of the gearbox where it plugs into. They are prone to snap/corrode on the wiring loom here due to the heat.

Also, check the fuses ontop of the battery - could just be a bad fuse

Cheers,

I changed the oil and filter last friday so I wonder if I've disturbed the sensor as the filter is close to the manifold? Is bank 1 referring to pre cat then? I changed the post cat sensor a couple of years ago so I knew the other one wouldn't be far off packing up!

I'm gonna plug in again tonight and see if anything has flagged back up.

Thanks again

Yes its not far off the oil filter. Its possible you have

Bank 1 is precat on these I beleive. I would have to cross reference the fault to ELSA to be 100% sure though.

alexperkins wrote:Hi bud

That fault is for the Lamda sensor pre cat ie the front closest to the engine. Its located in the manifold to the right hand side

Check the wiring on the front of the gearbox where it plugs into. They are prone to snap/corrode on the wiring loom here due to the heat.

Also, check the fuses ontop of the battery - could just be a bad fuse

Agreed... I had similar problem. Turned out to be a snapped cable connected to lambda sensor on the manifold.

Ben, Alex, thanks.

I think it's the lambda sensor on the way out as it didn't miss a beat today. I presume if it was a Brocken wire/connector it would be bad all the time or can they run ok with no sensor? It smells rich in fuel when I'm stopped at a junction and it only does 330 odd miles on a tank ATM! A visit to GSF this weekend me thinks!

You would notice a drop in performance and fuel wouldn't last as long

Only 330 miles. Christ how much do you normally get! I can polish a tank in under 300 lol

Don't get me wrong, I can empty the tank in 300 but on the normal commute to work from full to empty, I get about 360. Its a lose engine, its knocking on 144,000 miles now!

Thats impressive!

Maybe i should remove some lead from my foot

Quick update, I bought a new Bosch pre cat lambda yesterday from GSF. As I went to split the multiplug, I noticed that the car wiring was split. 4 out of the 6 wires were broke close to the plug. I ended up chasing the wires back into the harness and cutting the plug off so I could repair it on the work bench as access in the car was tight to say the least. 3 hours and lots of soldering later its fixed and the new lambda bolted in too. It probably didn't need the lambda but it can't hurt been a high mile car. I'm still getting poor readings on the VAGCOM readiness page though? How can you test the cat's performance? Take it to a garage? Can these cars pass MOT's with no cat?

Readiness page is nothing to go by - I would ignore that as it often lies.

If the lambda fault has now cleared it should now be ok.

And no, they cannot pass without a cat!

If it concerns you, take it to a garage that does MOTs and ask them to do an emissions test.

Alex, I'm going to run it for a week and see if the MPG, performance and fuel smell all improve. If not, I'll take it for an emissions check. I've got a complete spare system including cat so I'll change that next if its failing garage test. Cheers

Cool

I need a new center and backbox. Trying to find a genuine one in good nick is like gold dust these days

Result! I got 370 miles before I filled up. It worked out at 39mpg so I'm happy with that. I don't know if the cat is still doing its job but the car has got nearly 11 months left on the MOT so I'll worry about that when the time comes. Thanks to Alex and Ben for the wiring clues, that was without doubt the problem.

One happy Stan

Excellent news

No problem Stan - glad we could help