rh_wa

In testing my o2 sensor, I followed the FSM and tested the o2 sensor heater resistance using an ohm meter and get no readings whatsoever.

I've yet to do the o2 sensor feedback voltage test but is seems unnecessary if my o2 sensor heater resistance is nil, thus a shot o2 sensor.

Would that also cause a rich condition? Would this also cause a situation where after I've driven the truck, stop someplace to run an errand, and upon getting back in to start the truck, it runs rough and barely idles for about 5 minutes and then smooths out? Thanks.

Done so far:

new spark plugs (OEM)
new spark plug wires
distributor cap, rotor
lube, oil, filter
fuel filter
took out and ohm tested the engine coolant temperature sensor in a bowl of water (seems to be within specs)
TPS seems to test fine.

mt_goat

It's possible, the O2 sensor helps the ECU adjust the air/fuel ratio. How many miles on it? Its best to stay with Toyota OEM parts when it comes to sensors, plugs and wires.

rh_wa

Well, about 6 months ago, I threw a code (dash light). Took it to a local shop and they replaced my o2 sensor. I highly doubt they put in an OE type sensor, since it was one of those "we service every car" type of shop (including RV's and other big rigs).

After reading various posts, and especially seeing no readings in my ohm test of the o2 sensor heater resistance, I decided to order one from sparkplugs.com (Denso, OE type). It should be here in the next day or two.

Truck has 152,000+ miles on it. BTW, new distributor cap, rotor, plugs are OEM.

scope103

yes. The O2 sensor has to heat up to about 300degrees before it works; the heater circuit will get it there quickly, and hold it there at idle (where the exhaust stream may not have enough heat in it). In your case, it sounds like your open-circuit mixture setting is off, and it doesn't get back to where it needs to be until you've heated the O2 sensor to its operating temperature.

I've had good luck with after-market sensors, but mt_goat's advice is certainly valid.

abecedarian

Iif the heater circuit of the O2 sensor fails, the O2 sensor will trend towards the 'rich' end of the mixture if it can't be heated sufficiently by the exhaust gasses.
That is so that when they fail, you don't go lean and burn the engine up.

I have installed universal Bosch sensors (both of them) in my 91 and they've been there for nearly 3 years and have not caused an issue.