Chrysler Repair: 1995 Concorde engine check light: code 21, oxygen sensors, pvc valve

Question
It comes on after about 10 minutes from a cold start.
If I turn the engine off it doesn't come on again during that drive.
If I go on a long drive of 100+ miles this problem goes away, but returns after several days of city driving.
Engine check codes are 12 & 21.
Mileage about 90K miles. Engine running perfectly.
Recently replaced the PVC valve + rubber tubes and the small rubber tube at the rear of the injector frame.
TIA; Gerry

Answer
Hi Gerry,
The 12 code is unremarkable, just meaning the battery was disconnected sometime in the recent past. The 21 code matches with the way your check engine is functioning because it says that one of the two heated oxygen sensors is not working properly. There is one on/near each exhaust manifold. The engine controller doesn't evaluate those until it warms up, thus the delay. You could try reading the voltage (insert fine pins in wires: black wire and gray wire of the sensor proper) after the engine is warmed and idling. You should get a reading of around 0.5V. If it is either 0.1 or 1.0V or thereabouts, then the sensor is probably shot.
Roland