1999 Honda Civic check engine code PO 135

Car runs fine, but the check engine light has come on 2 times now and the code that comes back is “PO 135 - 02 sensor heater circuit malfunction, bank 1 sensor 1”. What the heck is this?

Your oxygen sensors have a heater circuit in them to get them warmed up quicker. An oxygen sensor will only work correctly when warmed up. The code is saying the heater circuit is bad. My shop manuals usually give me a method of testing the circuit to find the fault. Typically, replacing the sensor will fix the problem.

Are there multiple sensors and where are they located - i assume in the exhast manifold area? Thanks for your answers!

On your 4 cylinder engine exhaust, are the oxygen sensors… One fore, and one aft of the catalytic converter. The code PO135 is for the one in front of the catalytic converter.
The price of diagnostics is about the same as the price for a replacement oxygen sensor. You decide which way you want to go on this 10 year old car.http://www.autozone.com/N,16100117/shopping/partTypeResultSet.htm