Error Code 0420, Idle problems (after MOT)

[mikejd72]

Hi,

I have a Seat Ibiza Sport 1.4 2006, had the MOT start of June and passed fine. Only issue was a slight crack in the exhaust downpipe that was replaced. I had to go back the next day as it was rattling and had to banged back into place!

I presume it's connected but after the MOT when I turned the car on the RPM needle was bouncing slightly, from just under 750rpm down to near 500 then back up to just under 750 again where it would remain stable. The same would happen if I slightly revved the engine when stationary, also when coming to a stop. About 4 weeks later I had a good run in the car on a weekend away, 320 miles round trip and noticed that after this the idle issue stopped completely.

Yesterday the Emission Warning Light came on, popped into the garage today (different one as the usual seems to produce more faults then fixes if you know what I mean!), they ran a diagnostic check and fault code 0420 came up. They did an emissions test and the CO level was high (.31%, fails at .30%). This was on the first fast idle test. 2nd fast idle the CO went down to .08%. At the Natural Idle Test it was .49%, with the fail mark at .50%. They cleared the code, gave the car a good drive and the Warning light has not come back on. They asked me to give it a good drive for a few days to see how it goes, but I noticed the Idle issue has started again!

If it's not a blip then they think it's either the O2 sensor or the Cat.

Any one encounter this before and what do you think it could be? I had no issues like this before the MOT and the new downpipe going on.

Cheers,

Mike

 

[Arkaig]

Familiar story (56 plate 1.4 Sport here)

There are 2 cats on this car, a 'pre-cat' at the downpipe, and one further back in the usual place. It MAY be the second cat which is giving issues, or it may be the front O2sensor.

A similar thing happened to me after replacing the entire system from the downpipe back. I got about a week before the P0420 started. I replaced both front and back O2 sensors to no avail. The only thing which fixed it was to buy a small spacer on ebay for about £9 which fits between the rear O2 sensor and the exhaust. The light has never returned.

The thinking here is that any replacement exhaust parts which are not OEM can be of slightly different spec thus the O2 sensors are getting different readings from usual. The spacer brings them back within tolerance.

A caveat to all of this: it does sound like your car is running a bit rough, whereas mine was alright. This suggests the front O2 sensor may be on its way out, which also explains the emissions problems.

The rear sensor doesn't really do much btw, it just monitors the emissions levels and 'complains' with a P0420 when they don't look right. It's the front sensor which affects fuelling and thus emissions and running.

Hope this makes some sense!

 

[Arkaig]

Another thought just came to me. Since the front O2 sensor sits in the downpipe, it may not have been tightened properly when the new part went on, or maybe the cable wasn't connected back up right. It's free to check!