ECU code flashed 4 red and 1 green meaning air temperature sensor circuit malfunctioning. I checked voltage at terminal B and good at 5 volts. Then it says to check the continuity of the fuel tempature sensor circuit at terminal A. Do they(Haynes manual) mean of the air temp connector terminal A because I can’t find a fuel temperator sensor. They don’t say what to do if their is no continuity. I have already replace the air temp sensor but the check engine light is still on. I tried to clear the code but it is still saying air temp sensor. 1991 Nissan d21 ka24e
The 5 volts is the bias voltage from the engine computer. After the 5 volts goes through the resistance of the air temperature sensor, it will drop to a voltage which reflects the temperature of the air.
The continuity check should be just a check of the wiring, not the sensor.
You measure the ohms of the disconnected sensor; or, backprobe to read the volage when the engine is running. Or, remove the sensor, heat it to a known temperature, check the ohms, and comparre those ohm readings to the resistance/temperature scale.
You can disconnect the electrical connector from the sensor and check it with an ohmmeter. The resistance will vary based on the temperature the sensor is measuring. You can watch it by warming the sensor with your hand or cooling it with some ice. I have no idea what is normal for this sensor, but it should never read ‘infinite’, indicating the sensor is open, or very low-zero ohms, indicating a short. The fuel temperature sensor (if there is one) works the same way. I’m not sure what they mean by ‘fuel temperature circuit’ or if your car even has one. I’m admittedly a shade-tree mechanic, but I’d have to say that this wouldn’t be the first time I’ve seen a Haynes manual give cryptic, incomplete info, and the ones I’ve used have often been victims of bad proofreading, containing typos, drawings and photos with wrong captions, etc. Maybe a friendly dealer mechanic could help.