Sonic EGR System
Upon further research, I learned the following:
The Sonic EGR system is a closed loop EGR control system that consists of a vacuum-operated EGR valve with an EGR Valve Position (EVP) Sensor for feedback. The EGR valve is regulated by a duty-cycled EGR Vacuum Regulator solenoid. The PCM uses the EGR Valve Position Sensor for feedback control by measuring the position of the EGR valve pintle. On a Sonic EGR valve, the valve is designed such that the position of the EGR valve pintle is proportional to the amount of EGR flow, independent of the downstream manifold pressure. The PCM calculates the desired EGR rate and adjusts the duty cycle to the EVR to obtain the desired EGR rate by monitoring the EGR Valve Position signal.
As the EGR valve opens, the EGR Valve Position signal voltage increases. Normal EVP sensor voltage at idle (closed EGR valve) is between 0.24 and 0.67 volts.) The EGR Valve position signal only detects EGR valve pintle position. EGR flow is inferred from the pintle position. Actual blockage of EGR flow cannot be detected using the EVP signal. Therefore, cleaning the ports should have no impact on test results.
An electrical test of the EVP sensor is continuously performed. Did you have DTC P1400, P1401 or P1409 displayed? If not then the PCM read the EVP sensor voltage to be within range (4.81 to 0.048 volts).
After the vehicle has warmed up and normal EGR rates are being commanded by the PCM, the low flow check and a stuck open at idle check is performed. Since the Sonic EGR system uses EGR valve position for feedback, actual blockage in the EGR tubes and passages cannot be detected.
If the control system cannot open the EGR valve to the desired position, a P0401 will be set. The test is run if there is sufficient vacuum to open the valve, and the requested EVR dutycycle is high (>70%). If EVP sensor voltage: < 0.78 volts when commanded open then P0401 is set.
If the EGR valve position indicates that the EGR valve is stuck open at idle when commanded closed, a P0402 is set.(EVP sensor voltage: > 0.72 volts)
Since the sensor has been replaced, I suspect that there is now an electrical connection or wiring problem. Check the electrical connection and for broken or shorted wires.