I am having an issue with my 99 Crv. Swapped a gsr head on and vtec was not engaging with my chipped ecu. Go to read the CEL and realized the bulb in the cluster had been removed. I replaced the bulb and it was throwing the IAT code (10). I replace with a known good IAT, code is still present after clearing it out. I trouble shoot by measuring semsor resistance and then continuity between the sensor plug and the ecu, both check out. Then I measure voltage at the sensor plug and it is only 3.3 volts, when it should be between 4.6-5 volts. I measured the voltage with the sensor ground and a body ground. Then I measured the voltage directly at the ecu pin and it was only 3.3 volts. I swapped out for a stock p72, and the voltage stayed the same. I placed the stock b20 ecu and it stayed the same. When I hooked the scanner up to the obd2 port, it gave me the p0112 IAT low voltage code still. Seeing as the bulb was missing, it has probably been throwing the code since I've had it. Has anyone else experienced this issue? I am completely stumped as this makes no sense.

 

Trace the IAT voltage wire back the the PCM, and check that pin for 5V. If you still only get 3.3V the PCM is the only cause. If you get 5V at the PCM pin to body ground, you have a load in series with the IAT that is dividing the voltage, and resulting in the code. This is much more likely, because the 5V supply comes from the ECM, and all other 5V sensors pull from the same source. This would result in a DTC for every 5V sensor on the car.

If you do have 5V at the PCM pin itself, check for continuity between the ECM 5V pin for the IAT, and the 5V pin on the IAT sensor plug itself. If you get resistance over 2 ohms or so, this would confirm a series resistance in that circuit. This could be a poor connection somewhere (solder joint, etc), or a load spliced into it.