I have a Navien NCB 240e Combi boiler and it started throwing a 352 (water pressure) error.

Here is a blow by blow account of fixing it. I concentrate on draining and then purging the system as these are a bit glossed over on other videos.

Chuck Barron has a vid:    • Navien Tankless water heater Error Co...  
Elvis barbosa here:    • NAVIEN COMBI BOILER NCB- 180/210/240 ...  

in short - it is a simple fix.

The replacement sensor is a Navien 30014699A Water Pressure Sensor
461827 and I got it from tanklessparts.com ($48.66 inc shipping)

It fits there models:

CH-180-LP , CH-180-NG , CH-210-LP , CH-210-NG , CH-240-LP , CH-240-NG , NCB-080H-LP , NCB-080H-NG , NCB-150E-LP , NCB-150E-NG , NCB-150H-LP , NCB-150H-NG , NCB-180E-LP , NCB-180E-NG , NCB-190H-LP , NCB-190H-NG , NCB-210E-LP , NCB-210E-NG , NCB-240E-LP , NCB-240E-NG , NCB-250H-LP , NCB-250H-NG , NFC-175-LP , NFC-175-NG , NFC-175-NG-HIGH-ALTITUDE , NFC-200-LP , NFC-200-NG , NFC-200-NG-HIGH-ALTITUDE , NHB-055-LP , NHB-055-NG , NHB-080-LP , NHB-080-NG , NHB-110-LP , NHB-110-NG , NHB-150-LP , NHB-150-NG