Fault code P0420

Hi Arfur, the car did a round trip of 200miles & ran perfectly. The MIL has NOT returned even after short home/work runs since. I'm glad I didn't buy a new sensor as up to now no problems.

Unless it returns, I intend to leave well alone. INMHO perhaps I've disturbed the wiring or routing of it. Thanx for help.

Spoke too soon Arfur, just tonight the MIL came on, but car runs fine

Knowing what you do, is it time to buy the front sensor ie ahead of the cat?

Roland wrote:Knowing what you do, is it time to buy the front sensor ie ahead of the cat?

It would be the sensor after the cat (downstream) which would be the one I would change, if I couldn't do the correct diagnosis by looking at the waveforms.

But things can get expensive just changing bits without some testing, but it can be worth a try, sometimes.

If you think the Downstream sensor could be the issue, then I will remove it next. I will post a pic if necessary, to compare the two different sensors if anything found dodgy.

Thanx again Arfur

hello,
am new to the forum, and had my 02 C3 (also my first car, yikes) for about a month.
The engine light/diagnostics came on in first week, was the cat converter, so the garage replaced it under warranty (it ran fine with light on, then the starter conked out in rush hour, and it was possibly related so i took it in a few days earlier)

Been running well since, including a 260 mile round trip with no issues, but the light came on again a few days after.
The garage checked and it's still coming up as the cat (saying not running optimally, and to replace) even tho it's been replaced. he's tweaked it and light gone, but he's at a loss as to why it has come up again.

Have seen elsewhere that dodgy sensors can trigger the light coming on, anyone know anything about this?

Sorry am a bit of a novice, but first car and all that....

was only 44000 on the clock when i got it, and have added a few hundred to that already between trips out and work, garage chappy says it might have contributed to the cat fault warning last time, but replacing the cat should have stopped this.
this time, i noticed yesterday the cd kept flipping back to radio, which it hasn't done before.

Hi jayneey, welcome to the forum!

Did you also get P0420 on the diagnosis that was done before changing the cat? Its not worth changing the expensive cat if the errors are due to a faulty sensor, as Roland is doing.

jayneey wrote:Have seen elsewhere that dodgy sensors can trigger the light coming on, anyone know anything about this?

The MIL is brought up when a severe fault is detected by the on-board computer. The fault light could be due to many reasons and readings from a sensor (correctly reading or mis-reading) are the start. Diagnosis and fault reading is the next step, then changing parts and testing is the last.

If you have several problems, and you would like help with them, it would be best to post a new question in the correct section for each query. Starting a new topic for a new problem would be the best way to proceed

Hi,
Thanks for replying,
The garage chap said the first time it came on it could be the cat, so when it came on again he replaced it and am sue he said one of the Lambda (don't know if that's right; is it a fancy name for sensor?).
after last time it came on, he really wasn't sure and i think he mentioned changing the 'other' lambda, and if still a problem, he'll take it to the citroen garage. I'll be honest, am glad it's all going a bit wonky now while it's under warranty! it has only ever happened on the A47, for some bizarre reason, all three times the light has come on there.

Not sure of the code, but am splashing out a tenner on a OBD2 thing off amazon on payday, to try and keep an eye on my car and ee if it is just the same fault.

Also, at times it won't reverse, so started panicking about that (it's been a month, am still terrified of checking my tyre pressure.... ) till i read that basically, that's what manual gearboxes do sometimes....

In general i am happy with my funny little car and it's done me ok on many journeys! plus the added bonus (i think) it looks so funny, people don't seem to expect too much from it, and don't seem too bothered when i keep stalling it....

jayneey wrote:he garage chap said the first time it came on it could be the cat, so when it came on again he replaced it and am sue he said one of the Lambda (don't know if that's right; is it a fancy name for sensor?).

The lambda sensor is the O2 sensor, the ones found in the exhaust system.

jayneey wrote:Not sure of the code, but am splashing out a tenner on a OBD2 thing off amazon on payday, to try and keep an eye on my car and ee if it is just the same fault.

One of these USB OBD2 scanners should do the trick (if you have a spare laptop)
and one of these bluetooth OBD2 scanners if you have an android compatible phone.
If you have an iPhone, then the cost rises as you will need a WiFi OBD2 scanner because of the iphones inability to use bluetooth correctly




Citroen C3 fix engine light P0420

https://www.amazon.co.uk/gp/product/B00J ... 4D5ZQ2O7II

Am trying this fella, saw the bluetooth ones but brain can't cope with that level of high tech! If i could get a reading from tin cans and string i probably would.....


Thanks for the advice.

Also, i don't have a light on the dash when i dip my headlights? the blue symbol comes on for mainbeam, but nothing when dipped, just the spotlight? Is this normal? (it's my first car. i am terrified of it....)

jayneey wrote:Also, i don't have a light on the dash when i dip my headlights? the blue symbol comes on for mainbeam, but nothing when dipped, just the spotlight? Is this normal? (it's my first car. i am terrified of it....)

You should get the green light on the dashboard when the dipped lights are on.

Am trying this fella, saw the bluetooth ones but brain can't cope with that level of high tech! If i could get a reading from tin cans and string i probably would.....

Yes, the Maxiscan MS309 will do the basics for engine codes on the C3.