VTR Fault Codes / Lambda Issues - Saxperience

mrproper · 30th April 2011, 12:42

Hi All - first post, so please be gentle!

I've got a 2001 1.6 Saxo VTR, and put it through an MOT last month, and asked them to investigate the engine warning lamp that had come on about a week before. They told me it was the lambda sensor, reset the lamp, and said to come back if it came on again.

The car passed the MOT, including emissions, so I didn't think anything could be majorly wrong. However - the lamp has since come back on and won't go away, and I bought an ELM OBD-II code reader from eBay to save spending £50 getting a diagnostic from the garage again.

It reports the following two codes:

Quote:

Stored:
P0420
Powertrain: Catalyst System Low Efficiency

P0711
Powertrain: TFT Sensor Circuit Range / Performance

As far as I'm aware, the 2001 VTR has TWO lambda sensors, one on the top of the engine, and one just below the cat underneath/behind the engine.

Don't fancy shelling out £100s to a garage if this is a simple fix - as I could swap out a lambda sensor myself. What do you guys think? As it passed on emissions, I'm hoping it could be that the second lambda sensor is faulty, which would tally with what the garage said, and explain the 'Catalyst System Low Efficiency' reported? I don't fancy the cost of replacing the cat if it really is that!

Thanks in advance

mrproper · 30th April 2011, 17:22

Any ideas anyone?

I'm thinking I'll buy a new post-cat lambda sensor and see if that fixes it..

Is there a difference between the pre and post cat lambda sensors, except for the colour of the plug? I've found pre-cat sensors for £40-50 on eBay, but can only find post cat ones for over £100! Alternatively, where can I get part numbers so i know what to buy?

cheers!

Kyza · 30th April 2011, 19:35

Get the post cat sensor replaced and it should sort it. second hand one off here will do for £10.

mrproper · 7th May 2011, 19:04

Great, thanks for the help!

As I'm new I can't see the for sale forum without paying to be a premium member, but I'm a bit reluctant to do that if I don't know that someone is actually selling what I'm after...

If I type 'saxo lambda sensor' into eBay, loads of parts come up - some with blue plugs, some with green, some say they can fit pre/post cat, but some say they fit one or the other, but all say they are compatible with a 1.6 saxo vtr, even if the model numbers vary.

The part I'm replacing has a blue plug on my car, does anyone know a simple way to find out whether the parts available online will actually be compatible replacements - and do the pre/post cat sensors differ at all? Alternatively, does anyone have a working one for sale cheap?

Liam_ · 7th May 2011, 19:10

Is it a genuine Citroen CAT? If it's a cheaper one from a motor factors, they can sometimes give a low CAT efficiency fault code, even though the car still passes an emissions test.

If you want to try changing the lambda, then the post CAT sensor is probably the one to change. Should have a blue plug IIRC.

mrproper · 7th May 2011, 19:17

Hmm, I don't think the CAT has ever been changed, so it will be the genuine Citroen one from stock. I was hoping the CAT was fine and the sensor was just screwed (at the MOT the garage stuck the code reader on, cleared the warning light and told me it was "Lambda Coded", but didn't give me the precise code, bah!)..

My main concern is that the car is driving a bit rough until it gets hot, not sure a dodgy post CAT sensor could be responsible for that...

Liam_ · 7th May 2011, 19:20

The post CAT lambda doesn't alter how the car runs - it's only there to tell the ECU the condition of the CAT.

People who remove the CAT usually put the post CAT sensor in a metal sleeve so it reads 'clean air'. Others choose to just remove the sensor entirely and live with the light on the dash.

Might be worth looking into the coolant temperature sensor?

mrproper · 7th May 2011, 19:29

Thanks for the help... Maybe I should forget the lambda problem for now if, like you said, the second sensor won't be the cause of it running like crap, and look into other causes.

Bit suspicious of the "P0711
Powertrain: Transmission Fluid Temperature Sensor Circuit Range/Performance " fault code reading, as I don't think the saxo has that sensor... Maybe that's pushing it into limp mode hence it driving bad though..

axsaxoman · 8th May 2011, 11:31

your problem is possibly the downstream sensor ,but the other code is nonsense --you quite often fine cheap code readers do not have the software to suit all cars .
lets face it a bosch KTT diag machine is £7k + plus --
if the downstream probe is faulty fitting it to a sleeve will not turn the light out -wipe the codes and see how long before it comes on again.
could be cat getting old .
If it was cit /pug d8iag machine it would have given you different messages ,
like --cat againg
or downstream probe fault
heating circuit open circuit
signal wire grounded
not just rear sensor is not working
you get much more depth of info from a real tool
for anything other than simple faults cheap code readers are not good
the £50 you spent on the code reader is the price i would have charged for using the right tool and I would have looked at all the live data at the same time which them makes a good diagnosis much more likely

mrproper · 9th May 2011, 22:05

Thanks for the advice...

Bought the code reader from ebay for under £15, so still a useful thing to have around the house even though I doubt it's as good as the real thing!

Plan of action is to get a multimeter on the coolant temperature sensor, and as it's only £10-15, replace it unless it looks perfect. Then will look at pulling and sleeving the downstream lambda to see if it's faulty, then replace that if it doesn't put the light out. Then it might be time to get the garage to have a look!

Cheers guys

Liam_ · 9th May 2011, 22:48

With the right software, you should be able to view the live data for coolant temperature. I know my ELM327 does.

danimc · 3rd September 2012, 13:33

P0420: Catalyst System Efficiency Below Threshold (Bank 1)

Hi all first post above is my fault code i have changed lambda 3 times now and this time i have this code , i have 4 in 1 branch manifold also direct fit exhaust system, can any help me on the fault code thanks

Wee_monkey1987 · 3rd September 2012, 13:54

on your emission test what were the readings, my ecu light has been on for over a year with the exact same spec car, i've found the sensor isn't at fault its just a pre-warning PSA have written into the map to tell you your cat is due its end of life! replace the cat and job done - easy fix, cats for these i found vary from £60-£120 incl gaskets and flanges.

Danimc - the reason yours is on is because you've removed the cat entirely common fix is to sleeve the secondary lambda sensor

danimc · 3rd September 2012, 13:59

Cheers mate will have a look in to this as for sleeveing how is this done i have know idea?

On the emission test again im not sure as it was mot with the cat then i got my hands on it and stuck a 4 branch on this

Wee_monkey1987 · 3rd September 2012, 14:14

sleeving you can get from kamracing simple fabricated piece of metal that just goes round the lambda sensor cost about £5, simple to do remove the secondary lambda from the manifold, plug the hole with bolt or your manifold may have come with a plug use that, put the sleeve on the lambda probe element and cable tie to support it, job done! don't forget you'll need to switch off the ecu fault

danimc · 3rd September 2012, 14:33

Your a star mate, lambda sensor i have two one at top before the branch the the other below which on a down pipe is it the down pipe one

Wee_monkey1987 · 3rd September 2012, 17:14

Quote:

Originally Posted by danimc

Your a star mate, lambda sensor i have two one at top before the branch the the other below which on a down pipe is it the down pipe one

yeh it is the downpipe one you want to sleeve this only reads cat efficiency

vtr-man · 3rd September 2012, 18:32

I also have a 4 branch on to a catless straight through system. Its my mot next week and im fitting a sports cat into the pipe under the car. My question is this...do i not have to connect the lambdas?/connect only the rear one?/or do i need to connect both lambdas?. The car runs sweet enough without any lambdas without the cat so just would like to know who's done this and what results/combo is best? Many thanks.

vtr-man · 3rd September 2012, 18:32

forgot to add my vtr is the mk2 model!! cheers

Wee_monkey1987 · 4th September 2012, 10:21

Quote:

Originally Posted by vtr-man

I also have a 4 branch on to a catless straight through system. Its my mot next week and im fitting a sports cat into the pipe under the car. My question is this...do i not have to connect the lambdas?/connect only the rear one?/or do i need to connect both lambdas?. The car runs sweet enough without any lambdas without the cat so just would like to know who's done this and what results/combo is best? Many thanks.

you only need to connect the one lambda as your car with fail without it as part of the test includes lambda tolerance reading think its between 0.8-1.1volts, this probe sits BEFORE the cat!