The air/fuel sensors used in the manifold location in California spec cars (don't know what your standard is in Canada) are more complicated than the old "49 State" O2 sensors, and do get lazy without tripping the CEL even if the failure to "flip" is causing the engine to backfire and stall. This is a famous (except to the dealers) problem with 97-2001 I4 Camrys and usually happens when the engine has been run long enough to warm up, shut off, and then restarted shortly after. Doesn't happen in real cold or real hot weather, spring and fall are the most common. The cure is a new sensor, they usually last about 75 to 100 k miles, Denso and NGK make good ones, Bosch has been known to repackage NGKs.
The rear- after the cat- sensor is the one which looks for high emissions, and triggers the CEL, but they seem to last forever. Often the heater fails on the rear sensor before the sensor itself. This info may not help you, when these front sensors fail to "flip" they fail to flip from lean back to rich, which doesn't seem to be your problem, but it may be good info for others. When the engine first starts it runs in "open loop" for a brief period of time depending on temp, and ignores the A/F so sometimes the car runs OK for a bit and then gets into lean/stall. If you can keep it going it will warm up enough that engine temp matches the sensor input and the problem goes away until the next cycle. The strange thing is the engine will idle OK but stall as soon as the throttle is opened. Flooring it, (not a good idea in traffic) sends the system back into open loop and doing that for a few seconds will usually get the sensor working properly again, but, again, you can't do that in traffic. The first time this happened to one of our Camrys I made the mistake of taking it to a dealer, they replaced everything BUT the front sensor. I changed that myself after reading about lazy sensors. When the same thing happened with the other Camry I knew exactly what to do.
