We recently purchased a 1997 xj6 4.0l . The check engine light was on. I took the auto part store I work at and it coded po420 which was catalist effiency below threshold bank one. Bought one o2 sensor and did the guessing game as to which on to change. Lets back up here for a minute. When we bought the car the gentlemen we bought the car from said he did not put premium gas in it, just the low octane. When we filled it up my husband put premium it it and drove it for a few days to see if that would make the light go off. Of course it didn't. Ok now back forward, After putting the o2 sensor in we disconnected the battery to reset the code and the light went off and stayed off. My question is, what are the chances of hit the right o2 sensor or was it just the gas. This car did not have a run problem , smooth idle and very responsive.

 

Octane level of the gas has no impact on this, it was just a coincidence.

 

O2 sensors wear just like spark plugs, they do fail and flag codes. Disconnecting the battery would not put the light out either, so you guessed right

Wrong forum btw

 

fyi friend works for exxon gas companys are sham as ur probably no, you dont have to put in premium pretty often they fill up the premium lines with 87octane becuz the gas stationg will never no unless they have someone out there testing octane levels so save yourself a buck and just use the 87octane throw some octane booster in their every now and then to make yourself feel better

 

I guess working at auto parts for 14 years has it advantages, guessing. One problem down and several down.Broken door handle and non working steering column. Valve cover gasket, which I got from jag dealer and finding a power steering leak,antenna that won't go up and down unless you bang on it and blown speakers. Other then that it runs like a champ and looks

 

Not to rain on your parade, but how many miles on this Jag? It may simply be that you do, in fact, have a weak cat if you have 100k plus. If so, expect the light to reappear at some point with a P0420 again. I think replacing the O2 was a good idea, as these do wear out. I just suspect that if the O2 was the root cause of the problem, you would have gotten an O2 code instead. At any rate, you made it through in fine fashion! No matter what the issue was!