2001 Taurus, insuficient EGR flow

Just a wild guess, but I would check out the DPFE (Differential Pressure Feedback EGR) sensor. It measures the pressure drop across an orifice in the EGR tube, which is directly related to the flow. These sensors are known to fail frequently on the Focus (which I have) and I suspect the Taurus uses a very similar if not identical sensor. Ford has redesigned the DPFE sensor to make it more robust and less likely to fail.

You can backprobe the DPFE sensor output and make sure the voltage is within the normal range, I found some data on this at the local library on their AllData system. The voltage vs flow curves vary depending on exactly which sensor design you have.

There is also an EGR vacuum regulator valve that controls the vacuum seen by the EGR valve itself, and this component can also fail. It is driven by the ECU and it works by varying the duty cycle to raise and lower the amount of vacuum at the EGR valve.

All that said, you can take your chances with trying to do diagnosis by component replacement, or pay someone with the knowledge and equipment who can isolate the fault and fix it right the first time. I think if you have a MIL code stored and it is caused by the DPFE sensor, you should get it repaired for free at Ford. Do a google search for

Read your warranty book, especially the section on the emissions warranty. Mine says that critical emissions related components (and EGR and the associated valves and sensors certainly qualify) are warranted for something like 8 years and 80,000 miles. Be prepared to hear a boilerplate response from everyone at the dealership that 'only three things are covered, the cat, blah and blah'. I made the service manager read the relevant section out loud so he couldn't keep repeating that idiotic claim.

Good luck.