Q: What is the cost to replace oxygen sensors P0139, PO159

Hi there. In general, there are two O2 sensors on your Toyota, an upstream sensor (located near the exhaust manifold) and downstream (located after the catalytic converter). Based on the error codes you listed however, it’s not 100% certain that the sensors themselves is the source of your hard to start problem. In some cases, the electrical harnesses that connect the O2 sensors and relay data to the ECU can fail, causing the same codes to be triggered. You can request an estimate to have the O2 sensors replaced on your vehicle by clicking this link, but it would probably be better money spent to first have a professional mechanic complete a car is hard to start inspection first to determine the source of your symptoms so the right repairs can be completed.