Hi,

I am only new to this forum, but thought i should check it out. I have just bought a Fiat Stilo and i have a fault coming up with the following code, P0420.

Can anyone advise on what this problem is and what i should to fix the problem?

Thanks

Kenny G

Hi,
I had that problem as well (hope its solved...ill explain why)
that code means that either one (or both) lambdas has broke...cost 100 quid each...in he worst case scenario its a whole catalyst to be changed...but thats a few houndred quid...before taking such an action i would use CataClean.
You can really feel the differemce after using it and it might help you(as it cleans full exhaust system...it did help me for couple of weeks and then light came up again....so i have decided to change both lambdas...
why both? coz you cant check which one is getting wrog read.
i had that done about 2 weeks ago and its fine...hope it stays that way...

hope that helps a bit

Fault code P0420 = Catalyst System Efficiency Below Threshold

Has the car had misfire or the wrong fuel put in it?

Dave.

No, it doesn't mean that "either one (or both) lambdas are broke"
Stilo EOBD is smart enough to be able to pinpoint which lambda has a problem if there was one and tell you

As Davren says, it's spotted something that doesn't look right when comparing the pre cat lambda with the post cat lambda readings. A simple recurring misfire/ coil gone or exhaust leak can trigger it

Pre cat lambda should oscillate around every second when at tickover whereas post cat should be fairly steady once warmed up.

I have always put unleaded fuel in it (1.2.16v 2002). As i had only bought it, i took it back to the fella i bought it off and he took me around to the garage that does all the work for his company. The mechanic then put this liquid into the fuel and said to run the car for a good while as in a big journey and i did. I travelled for 70 miles nonstop to get home.

The the light came on a couple of weeks later. I have a friend who does all my mechanic work and he said he will look at it for me. Would i be better checking the sensors first or just replacing the cat?

And if it is the sensors, how do you determine which sensor is at fault?

Many Thanks in advance for all your advice, i really appreciate it.

If you re-read posts 3 and 4 you'll see there's nothing to suggest you need to replace anything. If either of your O2 sensors had a fault the EOBD would be flagging it as a fault and it would tell you which one was having trouble- but it hasn't detected any problems with them

It's just stating that at some time the readouts from both sensors didn't look like they should. The problem may be fixed, gone away -whatever

First step would be to clear the fault code and see if it reappears.


I had the same code over a year ago due to a coil breaking down, fixed the coil and haven't seen p0420 again since

Deckchair5 said:

If you re-read posts 3 and 4 you'll see there's nothing to suggest you need to replace anything. If either of your O2 sensors had a fault the EOBD would be flagging it as a fault and it would tell you which one was having trouble- but it hasn't detected any problems with them

It's just stating that at some time the readouts from both sensors didn't look like they should. The problem may be fixed, gone away -whatever

First step would be to clear the fault code and see if it reappears.

View attachment 71355
I had the same code over a year ago due to a coil breaking down, fixed the coil and haven't seen p0420 again since

Click to expand...

I had simular on my son in laws mitsi the other day fault code came up PO 421 but when I checked it in my code reader code book it said the same that davren said catylist system below threshold I put the light out and he went for an mot and past it :slayer:..10 mins later the light came on while on his way home.. we found that it was the downstream lambda to fault and the light has stayed off..

That's the way. You don't go around changing cats or sensors or anything until you find out what's causing the fault code P0420 which is more of an alarm signal.

I have replaced the old lambda with a new universal model. I have not clear the error and I do it tomorrow. The car runs god as before. I will check the diafrag with the software tomorrow.

Hi,
I had error P0130. I changed the first lambda sensor and then error P0420 and P0130. Whatś the problem with this car

Regards menola

You have a circuit problem on the lambda No.1 and not necessarily anything wrong with the lambda sensor. Check the lambda outputs and wiring and make sure the signal is getting to the ECU ( no wiring damage or poor connections)

P0420 is saying ECU isn't happy with what it's sensing at lambda No 2 but that will be because of the lambda No 1 not working properly in the circuit

Hello, I hope you can help. I drive a 1.6 16v Stilo, 2003. And I keep getting a 0420 fault-first occured every 4 days and after I replaced all 4 sparkplugs and all 4 coils and added 4 bottles of Engine cleaner to 4 tanks of fuel and changes the airfilter I noticed it only occured when driving around 120-130 KPH (around 80 MPH) w. my Engine rev. over 3500. It would flash (orange Engine symbol) and go away only to re occur after 2KMs of driving. Have measured coil values/response/voltage and Lambda 1 and 2 output and see no weird things. Than a friend reset to factory settings and after 2 weeks the orange light was back on, showing me the 0420 fault again, measured it with official Fiat scantool, lamda and coil values are OK. Please help.

Hi Chaps and Chapesses, thicko here. Thought I'd stick it on here rather than start a new thread. I have plugged a Gendan code reader into my Stilo and after a few seconds it reads 'SCAN ... PWM' and stays like that for ages. Does this mean it is actually scanning for fault codes (if so how long does it take?), or is it frozen? Says nothing about this stage in the process in the instuction manual.

Anyone more intelligent than me care to let me know please?

Thanks in advance.

Exactly which Gendan code reader are you using?

It's a GCR05 Shadeyman.

Re my post above;

Sorted. Gendan emailed me with how to get around the problem.

No fault codes showing.

Hmm. Could you please share that email with us . I have po420 and both lambdas has been changed...

Hi,
I have used a lamp and removed lambda1 I can see no damage on the top on calalyst. It looks fine in the end of catalyst. No dirt.

So catalyst is ok. Lambda1 has white colour and I have asked about it and they say that the engine get less fuel, poor mix. Could it be that fuel filter needs to be changed? How can you replace the filter, pictures?

/menola