Originally Posted by Old Man
That statement is totally in correct and now is becoming common place looking point for mechanics.
Trust me my statements are totally true. I've been working with oxygen sensors and GM engine management systems every day for the last 27+ years.
Thanks for the mini lesson, but I know exactly how they work.
However P03xx series DTCs have really only been around since 1996 and OBDII. These are the result of irregular crankshaft velocity variations as measured by the crankshaft position sensor (CKP) due to a significant misfire or mecahnical disturbance (even a rough road can cause this in a car with a std trans and the reason why most cars have a "rough road detection" algorithm with the help of the ABS wheel speed sensors)
The possible causes of a mis-fire sufficent to set a P0300 are of course numerous (including an erratic MAF) but the need for O2 replacement is more likely the RESULT of the misfire, than the root cause. The O2 sesor is essentially "ignored" by the engine managment system under higher loads (when PE modes take over) so the "lean detonation" as you describe would not occur based on any O2 input. The basic "narrow-band" O2 is essentially most influential at idle and steady state (light-moderate) road loads where it's primary purpose is to provide feedback required to maintain correct combustion stoichiometry (approx 14.7:1) neccessary to insure the catalytic converter is operating most efficiently in it's reduction/oxydation modes.
While it's true that a "pinned" O2 voltage can cause the engine to run extremely rich or lean, this is actually a very uncommon root cause due to the electro-chemistry of the sensor and typically only caused by a circuit fault (short to voltage or ground). Much more common however is a "sluggish" O2 sensor or one that no longer develops correct electrical potential across the zirconium boundary resulting in a "dead" O2 (resulting in a fixed 450mV) Another potential (and common) O2 sensor fault is a failure of the electrical heating element within the sensor.
The big thing to remember is the O2 doesnt techncially measure Rich or Lean, but the actual oxygen in the exahust.(that "might" indicate rich or lean) When a misfire occurs, the missing cylinder not only releases unburned fuel but also a "bubble" of oxygen at atmospheric concentrations (due to the lack of combustion) and the O2 sensor therefore erranty measures this as a false lean condition,(it cant measure the hydro-carbons) so even though fuel might be dripping from the tail-pipe the ECM/PCM "thinks" it's LEAN! The engine managment system therefore reacts by adding even MORE fuel, resulting in an extreme RICH RUNNING engine but with an indicated LEAN on a scan tool (voltages lower than 450mV on a scan tool)
If this was left unchecked for any length of time, the resulting raw fuel passing through the O2 has been known to "poison" the delicate chemistry within the zirconium bulb, destroying the sensor.
When an owner looks at their repair order (after bringing it in for a poor running condition and/or MIL) all they see the "Scan & diagnose P0300 DTC" combined with a "replace Heated Oxygen Sensor" and assume that was the root cause when more often it is actaully more likely to be "the fall-out"
Less informed service advisors/managers attempting to rationalize the repair order with the customer at pickup, might also mislead the owner.