1.6 AT petrol - power loss - P0420

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Hi guys!

I've been struggling with power for at least a year and can't seem to find an issue. Yesterday for the first time I saw P0420 after doing a scan which now points to cat.

While lack of power I could live with (to an extent) I also have issue where after driving 30-50 km I loose all power and I can't accelerate past 30-40km/h, switching ignition on/off seems to temporary solve the issue (like for 1-2km) and I have to repeat. This just absolutely driving me mad and the biggest problem I have to wait at least a month to receive a part from UK and that's the fastest option for me...

I've changed oxygen sensor after cat maybe 3-4 months ago, I've recently changed spark plugs/leads/coil. I'm attaching some screenshots perhaps someone could confirm cat issue or suggest me some other tests that would 100% confirm the issue so I could order right part and fix this once and for all?

PS: I already found cat "BM91275H" that ebay seller confirmed will fit my damned focus, anyone had tried to install that?

Thanks guys!





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  • Your long term fuel trims are high. Also bank 2 sensor  is mimicking bank 1. This maybe because the they are not hot enough or the cat is dead. ( did you use a cheap Chinese lambda?)  I would remove the cat and visually inspect it for blockage.

Taken from here

Another diagnosis which can be done using OBD2 system is determining the efficiency of catalytic converter. The output of downstream oxygen sensor, located following catalyst is compared with the upstream oxygen sensor located prior to catalytic converter. In the efficiently working catalytic converter, the upstream oxygen sensor will switch between lean and rich voltages, nevertheless the downstream oxygen sensor will have much fewer voltage switching. If the converter efficiency is poor (on account of contamination), the downstream O2 sensor output may be like the upstream O2 sensor output; which is the post cat oxygen sensor will switch more often between lean and rich voltage levels similar to the pre cat oxygen sensor.

Here is a pic from my car. Bank 2 sensor (pink) should be stable and no lower than .7 volts.



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Sounds like your cat has broken up inside, and partially blocking exhaust giving reduced power.

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Thanks guys. I've changed cat, but it wasn't cat. I still have sudden loose power where I can't accelerate past 30-40km/h (throttle to the bottom) after driving for 20-30 minutes or 30-40km and it's just driving me crazy (and my wallet). The graph improved slightly (see pic below), At 2500rpm the LTFT seems to go down to ~1-2% (I consider this an improvement, since before it would go up), this from what I understand would indicate vacuum leak.

I've googled around and the best solution recommended is smoke machine, problem is where should I pump smoke to (i assume I need to pull some hose from somewhere)? Alternative options were #1 propane (I couldn't find any, closest I've found 67% butane + 33% propane, but hardly usable because its for cooker or smth like that, the rest is 100% butane and I'm not sure if I can use that) and option #2 wd40 (have that, but not sure where to spray and what should I expect, RPM increase, decrease, stalling?). 

Local mechanic thinks it could be pump, after he first though its engine oil. Problem is they guessing and I'm paying, so I'm open to any suggestions in terms low cost testing to confirm actual problem (pictures would be helpful!)

PS: I have no clue if high LTFT is related to my sudden loss of power issue or its two separate problems, given my poor luck there might be even more underlying issues.

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