One of the most common issues that people need help with on the 6.7L Powerstroke diesel engine is diagnosing a code P207F. This is one of the harder codes that a Powerstroke can throw at you because this code doesn't actually point to a specific problem. This is what is known as a conditional DTC. Essentially that means it a diagnostic trouble code that only sets during certain conditions, not when a hard fault is actually detected. In the case of the P207F, this code is informing you that your SCR system is not operating correctly. The actual definition of the code P207F is "Reductant Quality". Its such a generalized code that literally means the SCR system is not reducing NoX output effectively and could be because of the quality of the reductant. Reductant is the diesel exhaust fluid or DEF.
The 6.7L Powerstroke PCM monitors the conditions in the exhaust system by using 4 EGT sensors, 2 NoX sensors, a DPFE sensor for the particulate filter, and a particulate matter sensor. All these sensors work together so that the PCM can control emissions output. The aim is reducing an emission called NOx or "Nitrogen Oxides". These emissions get produced under high cylinder temperatures which happen often during high load in a turbocharged engine. So anytime the engine is working, the system injects DEF fluid to chemically react with the NOx present in the exhaust. The harder the engine is working the more DEF gets injected to counteract the higher concentrations of NOx.
The PCM uses information from the front NOx sensor to determine how much DEF to inject, and it monitors information from the rear NOx sensor to determine how effective the system was. The P207F will set if no other codes DEF injection codes are present but the system still is not performing as expected.
If you have a P207F DTC, you may also get a message about Forced Limited Idle Only or Vehicle Speed Limited to 50MPH. Now that you know what is going on, you can start to try to determine the cause of the code. So many things can cause this P207F to set. When performing diagnostics you always start at least intrusive or expensive solution and work toward the most intrusive or expensive possible solution. I am going to lay out, in order, things you should be checking. Some of these do require scan tool access to be effective.
Engine Air Filter - Incorrect or restricted air coming into the engine can cause unexpected air readings and combustion mixtures. Be sure to check to make sure the filter is clean.
MAF Sensor - A dirty or damaged MAF sensor can cause unexpected air readings causing incorrect calculations.
Exhaust Leaks - Any leak in the exhaust can cause the NOx readings to be inconsistent. Any leak in the exhaust before the turbocharger can cause boost related concerns.
MAP Sensor - This sensor sits on top of the intake manifold and becomes clogged up with carbon buildup. Remove the sensor and lightly tap the carbon out. You can use a small pick to loosen the carbon if needed.
EGR Valve - Clogged EGR coolers likely wont lead to P207F, however a sticking open EGR valve can. Use a scan tool to verify EGR_A_POS is less than 1v or at 0% when EGR_A_CMD is 0%.
NOx Sensor - These sensors age and become biased. While driving the truck you should monitor both NOx1 and NOx2. Simply put, NOx2 should remain less than NOx1. They both should warm up and respond in a timely manner.
Exhaust System - A clogged particulate filter can cause issues however you generally get other codes confirming this. The SCR mixer or catalyst system could be damaged but this is a last resort because it is so expensive to replace.