my 2003 1mz-fe has developed some issues, only fault code is the front bank o2 heater. on tickover its tunning lumpy but notvmiss firing higher revers it seems fine not using more fuel than normal andnonloss of power. there a slight smell of oil burning but this is prob due to rear cam cover gasket leaking onto exhaust. ive changed the front spark plugs but never got round to rear ones as id have to strip the top of the engine for access. any suggestions?