How many Oygen Sensors are present in Honda Civic 2000? I had check engine P0135 and Firestone replaced 2 oygen sensors which costed me $500. Now after 2 months, I am getting check engine again. This time I went to Autozone to check the code. And it is P0138 which is related to Oygen Sensor. Is it possible that after replacing the sensor it has gone bad again? Or this is different sensor. Just wanted to know how many oygen sensors are there in the car, so that I can Firestone to do it for free since it is in warranty.

Thanks,
San

There are two sensors but there are different ways they can fail.
P0135 refers to the heater circuit of Sensor 1, while P0138 refers to the actual output from Sensor 2.
Those sensors can cost over $200.00 just for the part. Add diagnostic charges and labor time to replace the sensor and the cost goes up.

I have replaced both the sensors when I got P0135 code. Now I am getting P0138. Does that mean there is still problem even when I replaced both sensors. If it is related to the earlier problem then I can get it done for free at Firestone since it is under warranty.

Appreciate your help. Thanks

The diagnostics may have indicated that the sensor was setting a code, but the sensor is merely responding to how the engine is running. If the engine isn't running properly, i.e.: getting the proper air-fuel ratio, the problem might be fuel trim or problems with the Engine Control Module (ECM). The oxygen sensors only tell what is going on, not always why.
The first code, P0135 was a heater code. That means the heater for that sensor or the circuit was faulty. Since you replaced both sensors and now have a P0138, you may have inadvertently installed a faulty sensor #2.
This needs diagnosed by a competent tech locally. Where you have been taking your car may not be the best option for correct diagnosis.