Originally Posted by 1sweetride

Focusing on the issue of "check engine light on" with code P0161.
I was provided with some voltage readings for this sensor so I could measure against and compare.
Provided readings:
- O2 sensor 2/2 unplugged at sensor
- Key in the on position, engine not running
wire red/gn 12 volts (battery voltage) I measured 12 volts Voltage to heater circuit
yel/gn 0.45 v measured 0.44 v
brn 0.0 v measured 0.0
brn/gn 3.5 v measured 3.6 v PCM side of heater circuit

This would indicate the plug on the PCM is properly seated and making contact.
I reconnected the o2 sensor plug under the car and tucked all the wires in the proper location.
Check engine lamp still on, with code p0161
Then, accessed plug C211, passenger side foot area, connected one volt meter between pin 2 and ground
and a second volt meter on plug C211 between pin 4 and ground. This will be a direct comparison between both O2 downstream sensors (1/2 and 2/2)
Engine was cold.
Started the car.
Voltmeter connected to pin 4 showed a pulsing low voltage, at least as good as a digital volt meter can, while the voltmeter connected to pin 2 stayed at battery voltage (13 volts).
To me, I'm thinking the pcm is not pulsing the heater "on" for this sensor, but can't explain why the voltage measurements are near identical as a car with no "check engine " lamp.
Maybe @Gannyx or someone with more experience than I have could shed some light on this.
Wayne