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Diagnostics on P0420 code

MadMalx
MadMalx
in Ford Focus Club

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The engine management light has been coming on for a while now on my 2003 Focus (1.6 Zetec SE engine), and after a bit of searching the internet I have found this could be either the lambda sensors or the cat. I used my diagnostics software on the car and found that once the engine had gone into 'closed loop', the pre-cat lambda was switching but the post-cat was giving a steady reading. Problem is that after a few days, when the light had come on again, I re-did this test and found both the pre and post cat sensors were switching with a very similar pattern, indicating a failed cat. Am I right in assuming that the cat might be borderline failing hence giving me the different results?

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    • The Digestive Forum Roundup (24th May - 31st May 2026)

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      The Digestive Forum Roundup (24th May - 31st May 2026) We hope this email finds you well and enjoying your week. As always, our forum has been buzzing with activity, so we’ve compiled the highlights for you. Plenty of interesting discussions are taking place, especially around the Ford Focus and Fiesta models. Starting off with the Focus, many members have been discussing issues with the Emergency Call System, particularly in the Mk4 model. PLB raised concerns about persistent malfunctions, a se
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