Hello, I have had the problem since the end of 2009 that with my Ford Ka (built in 2006) the engine warning light is sporadic. The read error code is always the same: “P0420 – efficiency of the catalyst below limit value (cylinder series 1)”. Sometimes, however, the error occurs only 2500 km after reading again. Depending on the mechanic, I should have the Lambda probe or the cat changed or simply ignore it. One even talked about the fact that in the worst case, d It can explode engine (realistic???). In the forum and on some pages I have already read a little bit into the problem, apparently this is a well-known weak point at Ford. But since every mechanic seems to have a completely different opinion, I don’t really know what to do. Since the Lamba probe measurements are used to regulate the air-fuel ratio, can it theoretically come to consequences, or have I misunderstood this? I have just passed the exhaust test, but the mechanic told me that during this short measurement a malfunction of the cat could not be detected. Since I do not trust any of the mechanics at the moment, I hope that you can help me further (even if the topic “Lambdasonde” occurred here a few times already). Gruesse, Tanja (Delicately) I hope you can help me (even if the topic “Lambdasonde” occurred here a few times.). gt the spelling, on my English keyboard there are no umlauts..)