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Lets examine your readings first.
Code 154 means the sensor is not switching from lean to rich enough in 60 seconds. And there is no such thing as average voltage.
The O2 sensors generate their own voltage by measuring oxygen in the exhaust. They also have heaters that get power from a relay.
But a problem with the heaters should be a different code.
When the engine is running, the computer varies "on" time for the injectors based on engine temp, throttle position, and unburned fuel in the exhaust. So when the O2 sensors send voltage higher than .5 to the computer, the computer calls that RICH and shortens on time for the injectors. Then when the O2 sensor drops voltage down below .5 it reads LEAN, and the computer lengthens on time for the injectors.
This back and forth switching should happen several times each minute. Like the tick tock of a clock.
If the sensor(s) are slow to switch, or they stay on lean or rich for more than 30 seconds, you get the code 154. Each sensor has a different code for the same issue.
It could be a sensor problem, or a fuel problem, or something else.
Too lean can be low fuel pressure, too rich can be not enough air flow coming into the intake.
Oxygen Sensor Error (Lean Condition Indicated)
The ECM receives a signal from the O2 sensor based on the amount of oxygen in the exhaust. If there is excess oxygen in the exhaust, it is in lean condition and the O2 sensor voltage will be low. The ECM uses this information to adjust the amount of fuel delivered to each cylinder. Fault Code Description
Code 44 is for the O2 sensor staying on lean, and code 45 is for the sensor staying on rich. The sensor is supposed to switch back and forth several times each minute if it is working correctly and the computer is controlling the fuel mixture. The problem could be the sensor or a fuel problem such as an injector. The O2 sensor would need to be tested.
code 44 set on pre 1995 gm vehicles is a o2 sensor reading lean or bias to .2 volts or less. it could be the o2 sensor, it could be a vacuum leak making the exhaust have too much oxygen thus making the o2 sensor to read lean or put out a lean reading. If the o2 sensor is old 75k-100k it's probably time for a new one anyway. an exhaust leak near (upstream) to the o2 sensor can pull in air even though the exhaust is leaking out and fool the o2 sensor with the added oxygen also. you could also have a fuel delivery problem creating a true lean condition. there are ways a tech can test an o2 sensor to see if it's the sensor or a mechanical problem causing the code.
Code 44 indicates a lean condition as indicated by the Oxygen (O2) sensor, most times the cause is a bad Oxygen sensor and replacing it solves your problem. But sometimes your sensor is reading properly and your engine is experiencing a lean condition caused by a vacuum leak or low fuel pressure.
Trouble Code 44 indicates that the O2 sensor is showing a persistently high exhaust oxygen content (lean), despite the efforts of the ECM to increase ... Code 42 : Electronic spark timing (EST) circuit fault