o2 sensor p0150

Just replaced r/h fwd o2 sensor about a month ago and now I got a P0150 code yesterday.  Is something causing my dII  to run rich or what.  I don't see how the o2 could be bad already??  I am about to replace all of them (the other 3) and now this.  Front sensor RH bank stoichiometric ratio outside
operating band, what's a stoichiometric ratio???
thanks 

Hi robdis96
Stoichiometric ratio simply means the exact oxygen to fuel ratio that will burn without unburned fuel or excess oxygen left in the exhaust. Under ideal conditions, this is about 14 pounds of air to 1 pound of fuel. If your LR is an OBD II, code P0150 = O2 sensor circuit malfunction (bank 2 sensor 1) . This is the right side precatalytic 02 sensor. Make sure those connectors have "clicked" together.
How many miles and what year is your disco II?
Ed

As an 01 you are probably close to 100,000 miles and if so you should replace all 4 at the same time. What other fault codes did you have prior to installing one 02?
Mike

Thanks for the info, I replaced the fwd rh precat o2 sensor just after I had completed  other maint.  I was having the start issue  I had previously discussed on the  forum, I replaced the CKS along with the MAF.  I also repaired/replaced some gaskets for some minor leaks.  Once I replaced the CKS and MAF and clearing the codes all was good and then I got a P0150 code, I replaced the rh precat o2 sensor and all has been good until now.  I guess I'll check the connection and replace them all.  I am just under 100K.
The start problem did not give any codes.
Thanks