5E element With 5W 435MHz applied the 4431 meter Shows only 1W.
Indication with 100h element is normal. Attenuator is fully ccw. The 5E element frequency range (400-1000MHz) is different that the 100H range. The 100H frequency range is 2-30MHz so your reading may not be accurate. Make sure the element contacts on the 5W element are clean and making good contact with the element contact spring in the 4431. You can use a q-tip with a small amount of isopropyl alcohol to remove any tarnish, grease or dirt.
Do Bird elements determine overall accuracy of measurement? The element accuracy is +/-5% of full scale of the element, so with your 5W element you should be +/-.25W on your measurement and be in spec. If you are using a 100W element you can be +/- 5W and be in spec.
Is element faulty? When was the last time the element was calibrated? If it has been some time since it was calibrated you may want to consider sending it to our service center for calibration. To do so click on the following link and fill out the RMA Request. Once submitted, the service center will contact you within 24 hours with shipping instructions and a cost estimate.