# HO2S troubles **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** mazda5, mazda\ **Created:** [June 11, 2017, 7:53pm UTC](https://community.cartalk.com/t/ho2s-troubles/104859 "2017-06-11T19:53:28Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Aurelan](https://avatars.discourse-cdn.com/v4/letter/a/e8c25b/32.png) [@Aurelan](https://community.cartalk.com/u/Aurelan)\ **Post date:** [June 11, 2017, 7:53pm UTC](https://community.cartalk.com/t/ho2s-troubles/104859/1 "2017-06-11T19:53:28Z")
Last month I dealt with a few engine codes, including a P0030/P0031, HO2S heater circuit. The sensor was old anyway so I up and replaced it, all the codes wiped, we were good. Now I have the codes again, one month later. I checked the wires with a multimeter and they’re fine. I REALLY don’t feel like undoing tons of wiring( these Japanese cars aren’t very maintenance friendly). Any ideas other than ECM? ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [June 11, 2017, 8:25pm UTC](https://community.cartalk.com/t/ho2s-troubles/104859/2 "2017-06-11T20:25:29Z")
Bad O2 sensor maybe? Not unheard of. Also check for a heater relay or fuse that is used on some vehicles. I’d suspect a relay given the return of the problem as a fuse would not be intermittent. ---
**Author:** ![kurtwm1](https://avatars.discourse-cdn.com/v4/letter/k/13edae/32.png) [@kurtwm1](https://community.cartalk.com/u/kurtwm1)\ **Post date:** [June 11, 2017, 11:26pm UTC](https://community.cartalk.com/t/ho2s-troubles/104859/3 "2017-06-11T23:26:12Z")
unplug the sensor and check at the wiring for 12 volts at the heater circuit connector ---
**Author:** ![db4690](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/db4690/32/2583_2.png) [@db4690](https://community.cartalk.com/u/db4690)\ **Post date:** [June 12, 2017, 12:15am UTC](https://community.cartalk.com/t/ho2s-troubles/104859/4 "2017-06-12T00:15:40Z")
Don’t take this the wrong way . . . Are you sure you replaced the correct sensor? All modern cars have an upstream and downstream sensor And if you have a V engine, you’re going to have at least 3, possibly even 4, sensors And if you have a V engine, knowing your cylinder layout is critical. For example, if the code says Bank 1 sensor 1, do YOU even know which bank is bank 1 . . . ? And if you have an inline engine, do you know the difference between sensor 1 and sensor 2? Sensor 1 would be upstream, sensor 2 would be downstream. If’s actually quite common for the wrong sensor to get replaced ---
**Author:** ![Aurelan](https://avatars.discourse-cdn.com/v4/letter/a/e8c25b/32.png) [@Aurelan](https://community.cartalk.com/u/Aurelan)\ **Post date:** [June 12, 2017, 8:21am UTC](https://community.cartalk.com/t/ho2s-troubles/104859/5 "2017-06-12T08:21:34Z")
yeah Im sure, my model only has one upstream and one down stream. And I know it was the right one because when I replaced it, the codes wiped and the engine ran MUCH better. My multimeter has been giving me issues so got a new one. Tested again, and no resistance on the heater circuit. Dont buy cheap o2 sensors is the moral I guess