Hello.

 

I have a Skoda Fabia 1.2 HTP (BME) 64hp 2007 with 331500km/206000 miles.

 

Half year ago I got the engine light and faut code P0420 catalyst system efficiency below threshold bank 1. It runs quite well, a little bit missfiring at normal driving but nothing a normal driver should notice. But if is has been standing in the sun (for example when I am at work) it misfires a lot when I try to get up to speed.

 

My normal driving is 80km/h / 50mph roads to work 90km/55miles /day. If I drive it harder it goes better, with a longer trip with trailer the engine light turned of. 

 

The car past the inspection this summer but the CO-level was on the limit and they had to rev the engine  high for a while to get the emission at OK level.

 

I have fit new spark plugs, ignition coils and changed oil+filter without improvement.

 

Any suggestions what can be the problem?

 

 

I would change the pre-cat Lambda probe and the ECT sensor next, they'll be way out of spec. after 200k.

Hello Sepulchrave,

 

I changed the pre-cat lambda last year when I got high lambda-value at the inspection. The car started to missfire with the new aftermarket lambda and I changed back to the old one and car was running well again. Replaced the broken end silencer on the exhaust pipe and the lambda-value was ok. Trodo checked the lambda sensor against my VIN-number and it was correct but did not work. Shall i try with a new aftermarket sensor or is it safer to take one from the scrap yard?

 

I can se engine coolant temperatur with my OBD-scanner. I can borrow one IR-thermometer at work and check if it seem accurate. 

Hello again. 

 

15000km/9300 miles later I still have the same problem. It runs well when it is cold outside. When we had a couple of days with minus degrees the engine warning lights was not lit. If it has been parked in the sun a warm day it misfires a lot. It does not like hot weather and now summer and MOT inspection is coming. 

 

I have code P0420 catalyst system efficiency below threshold bank 1.

 

I have tried with a second hand pre-cat lambda sensor and a new engine coolant temperature sensor and a new contact for the temperature sensor because I had a small damage to the cables but it has not made any difference to the misfiring.

 

 Any suggestions? What is next step? Catalytic converter? But why is it running OK when cold outside?

I think you may have an air leak in the brake servo vacuum pipe.

Check that your fuel filter has not been incorrectly replaced with a 3 bar version. From recent experience, the BME engine uses a 4 bar fuel filter.

PM your VIN to me for checking if you find a 3 bar filter fitted (stamped into one end, it lives just in front of fuel tank, visible under car ahead of rear right wheel).

 

Edited 16 May, 20233 yr by Breezy_Pete
typo

Interesting! I will check tomorrow!

 

 

 

1 hour ago, Breezy_Pete said:

Check that your fuel filter has not been incorrectly replaced with a 3 bar version. From recent experience, the BME engine uses a 4 bar fuel filter.

I am said recent experience, as it seems these cars do not like having the incorrect fuel filter fitted due to the pressure regulator inside the filter.

You did have a different selection of fault codes, but it's a very easy thing to check, and fuelling could easily be imagined to be relevant here. 

1 minute ago, Breezy_Pete said:

You did have a different selection of fault codes, but it's a very easy thing to check, and fuelling could easily be imagined to be relevant here. 

 

Indeed, if you've got a small bit of mirror then you can angle it to where the markings are on the filter, without needing to jack the car up and crawl underneath.

I crawled underneath and the filter is marked 4 bars. 

 

I started the car and started bending the hoses in different positions and hoped to hear the motor misfiring but nothing. Could not see any damages to the hoses and the car was running good today,

I guess it is the hose on attached photo which is the brake servo vacuum pipe?

 

It will be 10 degrees warmer in the end of this week. I will check again when the temperature is raising against 20 Celsius.

 

Yes, that's the right hose to be checking, and yes, looks like the correct fuel filter is fitted.

Maybe try a compression test if you have a tester? 

 

Hello again.

 

I have removed and inspect the hose but can not find any cracks or damages. 

 

It has been warm and sunny here. The car does not run well after being parked in the sun and the engine still has not got up the running temperature.

 

If I erase the P0420-code it lights again after 100km, 

 

But last week I erase the code and was then driving with a trailer all week, total 500km (300 miles) half the time loaded with approximately 800kg (1800 pounds) and I never got the warning light. Even if it was a warm and sunny week and the car was misfiring when I left the hot car park. When I got all my free roof tiles home and started to run without extra weight and trailer the warning light came back again.

 

What can it be that makes the car runs better when the motor must work harder? Catalysator? Thermostat?

 

 

I do not own any compression tester.