Troubleshooting Help: O2 Edition [updated]

The 850 started throwing a CEL and I’m trying to figure out what it means. The code is P0172, which is Too Rich on Bank 1 Sensor 1. I checked my Torque App and I can’t pull an AFR reading, only O2 sensor voltage (I think the car only has 1 sensor for O2).

My drive is 25 mph for a minute or two, to get out of my neighborhood, then about 50 mph down the parkway for a few miles, then a lot of mixed driving, multiple stoplights with distances of a few hundred feet to several miles between them, mostly doing around 30-40 mph. So lots of regular driving under normal loads.

Here are my symptoms:

• Hard starting (may be unrelated)
• Car seems to run and drive okay all the time
• Engine comes up to temperature (according to dash gauge) in a normal amount of time (as I’m cruising down the parkway). I didn’t check the Torque App this morning.
• Curiously, the heater takes forever to warm up, like the engine comes up to temp but I don’t get any heat for a while after that, but then it blows very hot.
• The CEL lights up about 15 minutes after I leave, halfway into my drive. Car still runs fine. If I clear it, it stays off, until the next drive.

I’ve done some reading up on this, and the consensus among Volvo people is the P0172 is almost never an O2 sensor. But, the fact that it takes so long to throw the code even after the car is warmed up makes me wonder if the sensor heater is faulty or something. O2 sensors have a heater that is supposed to warm them up for a period of 2-3 minutes, then they should operate normally. Otherwise they take much longer to get up to operating temp. Perhaps the car is waiting for the sensor to warm up, based on expected heating of the element, and after a while it times out? Towards the end of my drive today I checked the sensor voltage on the Torque App and it appeared to be normal (I didn’t think to check it earlier).

The Volvo folks say the 2 most common culprits are a dirty MAF or a vacuum related issue. This is possible but seems unlikely to me, I serviced the Flame Trap (PCV) earlier this year and replaced a bunch of bad vacuum hoses, and had the MAF out and cleaned it. That being said, it’s still a possibility I missed a bad hose or something.

The last thing, and I just caught this randomly in a forum, is a bad Engine Coolant Temp sensor and/or thermostat. During warmup (when the O2 sensor is non functional because its still being heated), the engine adds a little extra fuel to be safe, like a choke on an old carb. It’s possible that one or both are bad, which is causing the car to always think it’s cold, thus always adding extra fuel. And if the thermostat is at fault it could also explain why the car is slow to get heat.

My plan now is to setup Torque this afternoon with a display for O2 voltage, coolant temp, and MAF readings and monitor them on the way home. Maybe I’ll even try the datalogging function, if I can figure it out. I’m off tomorrow so I should have some time to check vacuum lines and clean the MAF again.

Any other thoughts or crazy ideas?

Update: Opponaut Cmill below mentioned his XC90 did the same thing, but only on winter blend fuel. That brings up a very good point, and detail I overlooked. The last time I drove the Volvo was in early September. We have annual inspections in VA, and mine was due. It failed due to the parking brake being out of adjustment, and I didn’t have time to fix the car and get the re-inspection done before I left for a month long work trip. I did it upon my return and then had to wait 2 weeks for new tag stickers. So I just got those on and started driving the car this past Sunday. The only issue with this theory is it didn’t do this last year.