Toyota Repair: code 25 toy 4RNR 22RE, engine misfire, oxygen sensor

Question
Hi Ted,
My 91 4rnr (California) with 96K miles check engine light goes on for code 25.  O2 sensor checks out OK. Replaced distributor cap, rotor & plugs.  No loss of performance when light goes on when driving on freeway.  BTW, code 43 is also present even though I have the 6+V going to the computer at starting.  Any advise?

Thank you, Dan

Answer
This is an oxygen sensor code, the system is running too lean meaning that the sensor sees too much oxygen in the exhaust gas, this normally caused by a leak in the intake system or a possible exhaust leak upstream of the sensor, it can also be caused by an engine misfire or a leaking valve or blown headgasket.
The first thing I suggest is that a compression test is done on the engine.
If that checks good look for an intake air leak or an exhaust leak or anything that may cause an engine misfire, have it checked for a possible blown headgasket.