I've been having an engine check light for a while. It comes on and stays on for like 2 weeks and goes away, then comes back on after another 2 weeks. I'm thinking I might have fouled my O2 Sensor. The engine check light code is P0131 - O2 Sensor Voltage Low. I took my car back to the tuning shop today and according to the shop owner, he says the o2 sensor checks out ok. He said a possible reason for seeing this probably because the engine could be running lean at times. I'm running out of options and would like expert opinions from forum members. The car runs fine otherwise... regardless if the check engine light comes on or not.