23 - Intake air temperature sensor out of range, low
You can find the fault code definitions here:
https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method
The last suspect is the sensor itself. They have just one component inside, and failures are very rare.
This wondrous story is for the coolant temperature sensor. The intake air temperature sensor is less complicated, but it works the same way. This type of sensor is fed 5.0 volts from the engine computer, then it draws that down toward 0.5 volts as temperature increases. For theory of operation, the acceptable range of signal voltage is 0.5 to 4.5 volts. Anything outside that range triggers a diagnostic fault code. It is almost impossible for a properly-working sensor to draw that 5.0 volts down too little, or below 0.5 volts.
If that voltage supply wire is open, (cut), the computer will see the full 5.0 volts. That will set a "voltage high" fault code. To set a "voltage low" fault code, the signal /5.0 volt feed wire has to be grounded. That can be the wire rubbed through where i is laying on the sharp edge of a metal bracket, it fell down onto hot exhaust parts, or more commonly, there is corrosion between the two terminals in the sensor's connector. The second terminal is the ground return wire. Ground wires normally have 0.0 volts on them, but sensor ground wires go to ground through the computer, and there's monitoring circuitry in there. As a result, you will find close to 0.2 volts on sensor ground wires.
In some cases it is also possible to set this fault code when the voltage is between 0.5 and 4.5 volts, but a low voltage is indicating a hot engine, and the computer knows the engine has not been running for many hours. With a cold engine, the computer expects to see around 4.1 to 4.3 volts, and it should drop as the engine warms up. Even though, lets say, 2.0 volts, is an acceptable value, that could represent 160 degrees, and the computer knows that is not possible with a cold engine that was just started.
Most 1995 and older computers were not that sophisticated. They just set fault codes based on voltage readings, not from adding logic to the equation. You can, however, get a code when the problem is intermittent. In fact, those codes are the only way to know where to start looking when you do have an intermittent problem. If you have a scanner, your job gets a lot easier because you can view live data and see the sensor voltages the computer is seeing. If you do not have a scanner, use a digital voltmeter, and back-probe the signal wire at the sensor. Poke the probe through the rubber seal around the wire. These readings are only valid when the sensor is plugged in. If you try to read them with it unplugged, you are going to find the full 5.0 volts on the signal wire, and 0.2 volts on the ground wire. The ground circuit is shared by a number of other sensors, so that normal 0.2 vols will be there.
If you find the proper voltage, around, oh, 4.0 to 4.3 volts when the engine is cold, monitor that voltage as the engine warms up. Normal operation is to see the voltage decrease to around 2.0 to 2.5 volts, and maybe even less, by the time the engine reaches around 150 to 160 degrees, then don't be surprised when you see it suddenly pop back up to more than 3.0 volts, then start to decrease from there again. The computer has inserted a different series resistor to increase the accuracy of that range of temperatures. Even though the voltage jumped back up, the temperature values displayed on a scanner will continue to increase smoothly and steadily as the engine comes up to full operating temperature.
The range of voltage readings for air temperature sensors is much smaller, so you will not see much of a change as the engine warms up, and they do not use that two-stage circuit like coolant sensors do. The 0.5 to 4.5 volt range is still the acceptable range, but the computer knows when the engine has been off for many hours, the intake air temperature sensor and the coolant temperature sensor had better be reading the same temperature. The air temperature sensor's reading is important when the computer calculates how much fuel is needed for starting the engine. (Think of the temperature-controlled spring on the choke plate on a carburetor). The wrong reading can cause hard starting with a long crank time, and it can cause hesitations once the engine is running.
It is good practice to take voltage readings at the sensor before you ever unplug it. You want to see what it is the computer is seeing and responding to before you disturb any potential corrosion or bad connection between the terminals. Once you get the initial readings, and have a starting point, you'll know when something you did caused a change.
SPONSORED LINKS
Thursday, August 31st, 2017 AT 5:17 PM
