Thermostats keep the engine hot enough to heat cats. If a thermostat is not working properly, at highway speeds the engine runs cooler but won't set codes. A thermostat maintains engine temp constant for NOx burning. The instrument gauge doesn't swing enough to see temp changes. For it's age, a new thermostat is good maintenance. Does it have OEM plugs in it? Or updated single filament iridium plugs? Does voltage swing up and down on front O2 sensors. Voltage should swing. The rear O2 sensors should read a more constant voltage. Some scanners can read cat temps. Are both cat temps close in readings?

@EODguy
I don't think I've ever seen a O2 heater code on mine, should be a lot easier to track down than a mixture code. I'm pretty sure there are some good threads here on that.

I've never seen an O2 heater code on my '01 (not sure if it even HAS heated O2 sensors), either. Last time I got it inspected, I was able to get all the emission monitors to set, except for EVAP. Having one monitor not set is OK for my year model in Texas, though. It only took me a 5 mile drive (not following any "drive cycle" regimen) to get the others to set.

On the other hand, a few years ago, it took me almost 500 miles of driving to get my '04 Chevy 2500HD (6L gas) to set the monitors. Even after I closely followed the prescribed "GM Drive Cycle" at least 10 times. I drove it several days, with many key on/off cycles, but with no luck. Finally, all but the EVAP monitor set. I had replaced one downstream O2 sensor, and the evap filter, prior to that episode.

On other cars & trucks, resetting the emissions monitors has usually taken about 50 miles doing the GM drive cycle, after repairs and code clearing (most of my vehicles have been GM). The quick reset of the monitors in the X5 was a pleasant surprise. Can't wait to see what my wife's Lexus will do, when/if I need to reset the monitors.

Sent from my SM-G965U using Tapatalk

It does. There will be four wires going to each sensor - two of these are the heating element.

Yep, the heat circuit has a seperate ground that I think often gets overlooked so probably a good thing to check if one has heater code issues.

The EVAP one always takes the longest. It has to be above 37° Fahrenheit and you have to let it stand overnight and it has to have either like a full tank or empty tank or something. It's taken me forever to get it to turn on on the f30. E53 it still took a few days of random driving. The cycle they mention in ISTA, doesn't work. Even if all the sensors and everything are tip top it doesn't work.

Thanks all. I managed to get temp. tags today good for 30 days, so I can actually DRIVE the thing without worrying about getting pulled over, MUCH easier for trying to get the last codes to clear!

What *SHOULD* the coolant temps be for "normal" driving? I know the temp gauge is borderline useless, but I'm driving with a live OBD2 readout on the phone so I'm normally seeing it fairly steady at about 201 F.

I also replaced both front O2 sensors yesterday during my lunch break, haven't had a chance to get under the car to deal with the 2 rear. Front sensors are cycling heavily when I log them, rears are relatively steady at 0.4-0.5.

Now that I have the "luxury" of the 30 day tags, I'll probably drive it for a day or two, see how it does, and then replace the rear o2 sensors and clear any codes and re-start the process if needed.

Buy a cheap gas cap. Take the o ring off and put it on. Or flip your gas cap ring over. I've done this to several cars for EVAP codes. If the gas cap o ring is good enough, it will work to tighten up the seal. Flipping the o ring on mine worked for a while. Eventually, I took a seal off a new gas cap. No EVAP codes have appeared for two years.

201 is right on the money. It will move up and down between 198 and 203. And when you get it on the freeway the system will turn the t-stat heater on and the temp drops into the 180's.