I went to get a smog check recently and My car wasn't passing the I/m readiness check. specifically the HRT (OXYGEN SENSOR HEATER MONITOR), everything else was coming up green. I replaced the upstream o2 sensor a few months back and hadn't been driving much in the pandemic so I assumed it was a millage problem. After a hundred or so miles on and off the freeway I still had no luck. Since then I have replaced both o2 sensors again, driven more, still no green. thought It might be hung up on something so I disconnected the battery, everything was reading red, did about 80 more miles on and off the freeway and everything has cleared again except HRT. Not sure what to try next, please help.

 

Is the check engine light on? Is there a diagnostic code that can be pulled?

Most states will allow a pass if a single monitor reports as not ready. If I may ask, what state are you in (other than confused)?

You noted you changed out the O2 sensor, only oem, ntk or denso come recommended. Bosch seem to hit or miss. Have you inspected the O2 wiring harness for rub-throughs or breaks?

 

Thanks for the reply Goose! No trouble codes currently, I did have a o2 sensor bank code (can't remember, but I can look around to see if I wrote it somewhere) several months earlier which is when I replaced the upstream o2 sensor. No new codes, but that seems to be what has lead me to my current problem.

I'm in California, and it seems we can only have the evap monitor fail here.

I had an abagail sensor in it, and replaced that with a denso this last week. I checked the wiring to the best of my novice abilities, no obvious stress or damage there.

It still had the original o2 downstream sensor which I hadn't bothered replacing after the codes didn't repeat, but I changed that out in the midst of all my troubleshooting. I'm wondering if that might still be a problem since the one I replaced that with has since changed its amazon page to say its not recommended for my model. still the lack of any engine light makes me think its working fine...

 

Did a little checking on the o2 heater monitor. Appears to be a measure of voltage more so than actual operation. that is, if there is enough voltage flowing the heater must be good.. Take a look at the ground points paying special attention to the ones on the driver side inner fender. A poor ground point could reduce the voltage enough to keep the monitor from setting. Also take a look at the connectors to the pcm checking them for corrosion.