PO138 Error Code - lost!

Hey guys - my trucks giving me a po138 code and I just failed my emissions test(Ontario Canada). Does anyone know specifically what this code is relating to? I have a feeling it may be an O2 sensor but before I spend 80 bucks on one I thought I'd check here first. Tx

I just did mine a couple of months ago. O2 sensor, Bank 1, sensor 2. That would be the one after the converter. I found one for about $70. An O2 sensor tool would be worth the investment.

I just did mine a couple of months ago. O2 sensor, Bank 1, sensor 2. That would be the one after the converter. I found one for about $70. An O2 sensor tool would be worth the investment.
Tx HemiDaze- I was hoping someone knew which one it was to avoid buying both. I was guessing the one before :rolleyes:
Saved me $75