P1635

P1635- 5 VOLT REFERENCE 1 CIRCUIT.
DESCRIPTION- the PCM uses the 5 v. reference 1 circuit to feed the throttle position sensor, MAP sensor EGR position sensor and the fuel tank pressure sensor with 5 volts. The engine computer monitors this voltage while the engine is running and will set the p1635 code if this 5 volt reference is out of its parameters.

Well the code reads to be a low voltage, reference signal from some sensor to the ECM. Unfortunately I can't tell you what sensor it is. A scan-tool is needed to look at the data and find out which component is being affected.