What do the 131/151 codes mean?
The codes are specifically listed as P0131 and P0151 codes. The description in the Service Manual is:
P0131    Front O2 sensor low or engine running lean
P0151    Rear O2 sensor low or engine running lean
The key point to look at here is "O2 sensor low" when analyzing what is happening with the XIED's in place. The O2 sensor produces a 0.0 to 1.0 Volt output. Any voltage lower than a preset threshold in the ECM can trigger the 131/151 errors. Lower voltage equates to a leaner fuel mixture in the eyes of the ECM. The XIED's take a higher voltage from the O2 sensor and "shift" it to a lower voltage for the ECM to read. In some circumstances, the ECM will see this low voltage as "too low" and trigger the code despite the fact that the engine is running rich enough. The IED's take advantage of this voltage shift to richen the mixture.
So the engine can be running rich and the ECM might still trigger the "sensor low" signal.
There are also other reasons that the ECM will trigger the codes that we will try to cover below.

Will these codes hurt my bike?
No. These are historic codes and are not active. As long as these are the only engine codes you have and you do not have an active Check Engine light on, they can be safely ignored.

Do all bikes get these codes with IED's installed?
No. Many bikes never see these codes. You could also have the codes and never know it. Unless you check the historical codes on your bike, you might never see these.

Do completely stock bikes get these errors?
Yes. Completely stock bikes get these 131/151 errors on a regular basis as evidence that Harley has included a note to "clear and ignore" these codes in their Service Manual.

Don't historical codes mean there is a problem?
No. Historical codes only mean the ECM saw something it did not expect. If the code is considered a potential problem or there is a risk of damage to the engine, the ECM will turn on the Check Engine light. No such situation exists with the 131/151 codes. The Harley Service Manual actually notes that these codes should be "cleared and ignored".

I have read that these codes cause the ECM to revert back to open loop mode. Is this true?
No. The ECM will only revert to open loop operation if there is an active error code. Active error codes will cause the Check Engine light to come on. As long as you have no CE light, your bike will run in closed loop for normal riding situations.

When I take the XiED's off my bike, the codes go away. Doesn't this prove the XiED's are creating the problem?
No. It can be easy to confuse the installation of the IED's as the cause of the error. This is a normal reaction and quite understandable for the average rider. In engineering circles there is something known as "root cause" analysis. The "trigger" mechanism is not always the cause of a problem. In this case, the "trigger" is the IED's. The IED's do make the ECM more prone to showing these codes, but there are a host of issues that cause the codes. Harley has documented many of these causes in their Service Manuals. A copy of the specific section of the Service Manual is listed below.

Are there other things that can cause the error codes?
Yes. We have information from an EPA testing laboratory that there are several other issues associated with these codes. These are engineering issues that are deeply embedded into the HD Delphi ECM. Our information also indicates that Harley is aware of these issues, but does not have plans to create any upgrades to eliminate know problems.

Can you get rid of the 131/151 codes?
Maybe. It is important that a rider ensure that they have eliminated the common reasons for triggering the codes with the IED's installed. The primary triggers are exhaust leaks and loose O2 sensors. Even the smallest exhaust leak can trigger the codes. We have found that many of the bikes that have codes will get rid of them when they carefully check their exhaust system.

Can I just run my bike with the codes and not worry?
Yes. As long as you do not get an active check engine light, you can run your bike with the IED's. For any active check engine light, you want to have that checked to find out what is setting off the CE light.