This refers to the second oxygen sensor on Bank 1. Bank 1 is the same side of the engine that cylinder 1 is on, the very front cylinder on the drivers side. It could need to be replaced, or there could be a short to voltage on the O2 signal circuit, or it could also be that the connector has been corroded, and needs to just be cleaned. I believe this code is the same for all vehicles, not just Toyotas.
There are several possible causes for the engine codes P0136 and P0141. Some possible causes for P0136 are oxygen sensors, fuel injectors, or intake air leaks. Possible causes for P0141 are the engine control module and the oxygen sensors.
Trouble code P0136 means: O2 Circuit (Bank 1, Sensor 2)
Trouble code P0136 means:O2 sensor circuit malfunction (Bank 1 Sensor 2)
Trouble code P0136 means: HO2S-12 (Bank 1 Sensor 2) Circuit Malfunction
Trouble code P0136 means:O2 sensor circuit malfunction (Bank 1 Sensor 2)
Trouble code P0136 means: O2 Circuit (Bank 1, Sensor 2)
Code P0136 is the code set for your O2 Sensor that is located after your Catalytic Converter.
The code thrown will tell you which one is bad. Codes p0130 thru p0135 all refer to the upstream sensor under your coil packs. Codes p0136 thru p0141 all refer to the downstream sensor under the van.
I'm not exactly sure about the 136 and 141 codes, but the 171 and 174 are symptomatic of a known problem with Windstar 3.8 litre engines. Those codes combined with rough idle are prime indicators of a leaking upper intake manifold involving worn out "isolator bolts". See "Related Questions" below for more
O2 sensor circuit malfunction (bank1 sensor 2)
Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
O2 sensor circuit malfunction, Bank 1 sensor 2. Check the O2 sensor after the catalytic converter.