Hello, I have been trying to straighten out a problem with my wife’s 2003 Tacoma 2.7 4x4 auto. The thing has been running great for 122 thousand miles. Serviced regularly set a cel and scanner asked for front 02 sensor installed toyota 02 sensor light went out thought that was it. Upon test driving it I noticed an annoying surge at all speeds, not enough to make the tach jump but it is annoying installed set of plugs seals etc. on coils new fuel filter cleaned throttle body ( it was not dirty) bench tested IAC . Idles fine, Checked vacuum and fuel,pressure,Checked vacuum hoses and engine/body grounds no cel seems to be worse sometimes. I have even changed fuel pump and screen used snap on scanner says everything is working correctly,I am at my wits end, any help would be greatly appreciated.
Jack

 

IIRC, and I am often wrong, but the front, before the cat(s), sensor is an A/F (air/fuel mixture) sensor. The one AFTER the cat(s) is the O2 sensor.

Are you sure you installed the correct sensor? just double checking.

Also, ONLY use Denso or similar sensors, NOT Bosch. Bosch is designed for European vehicles, and, while it won't trip a CEL light, just doesn't provide the exactly right voltages to the ECU. Go with OEM whenever possible. For everything!
Make certain the wire feeding the sensors are in good shape. No corrosion on the connectors inside the plugs. Clean, bare metal is vital.
I always, ALWAYS, put a dab of silicon dielectric grease into any female plug I open then connect again. Keeps the crud and corruption floating around in the engine compartment from damaging the pins inside the plug. I do it to spark plug cables, distributor cap connections, headlight connectors, you name it. It really does help a lot.

Something to check also, is the TPS is properly aligned? No broken wires in the plug, correct ohm readings throughout it's throw, the idle switch sets properly, etc. Something worth a check. The FSM has all the procedures, and correct ohm values to read.

Hope this is all some help...
Pat☺