Went to my local Autozone to get the fault code that lit up my check engine light (Can't afford an OBDII scanner). Got "PO172 System too rich (Bank 1)". So, what would cause the computer to decide to make the fuel mixture too rich? O2 sensor going bad possibly? And what is Bank 1, Bank 2, etc.? Thanks in advance for any help.

Look for high fuel pressure (bad regulator), leaking injector or malfunctioning engine coolant temp sensor. Bank 1 and Bank 2 refer to the cylinder groups on V6 or V8 engines. While the rich mixture may be due to a bad oxygen sensor, there are several specific codes related to the oxygen sensors that may be more likely to show up if the sensor(s) are bad. See volvospeed/bay13 for detailed info on them. These codes distinguish front from rear sensor and refer to them as front HO2S and rear HO2S. How are the performance and fuel economy and is the car hard to start, particularly when engine is already hot?

The post was for a 96 850 which has a "straight" 5 cyl engine, not a V6 or V8. The info about Bank 1 and 2 referring to cylinder groups on the 850 is therefore incorrect. ECT sensors are suspect in a rich condition while FPR problems aren't that common. Fuel pressure is easy enough to check and rule out.

Bank 2 codes are a generic definition within various OBD-II protocols, the signals being null or not used on cars with an inline engine. Bank 1 can mean either "Bank 1 rather than Bank 2" or "this car has only one bank". My Ford has a V6 with bank 1 (cyls 1-3) and bank 2 (cyls 4-6). Each has a separate front and rear oxygen sensor which can generate their own codes. Several other codes on the Ford but not the 850 are bank-related. See http://www.obd-codes.com/trouble_codes/index.php.

Lets try this again. The question the poster asked was pretty straight forward and was specific to a 5 cylinder 96 850, not a V6 Ford or V8 whatever. The info you give is certainly correct but irrelevant to his car and confusing as well. When someone asks me the time, I just tell him rather than explaining how to build a watch ;)

Talked to my indie. He said, as "bl" stated in another thread, bank1 is the front O2 sensor, and bank2 is the rear O2 sensor. However, this generic code does not mean the O2 sensor is bad, just the part of the system that is involved with setting the code. If I understand everything he said, the bank1 sensor is associated with direct fuel mixture, etc. while bank2 (rear O2 sensor) is associated more with the air pump and the warming up of the engine. The guy has worked on volvos and saabs (only) for 20+ years. Basically, it could be anything from a malfunctioning injector to a TPS, etc. Thanks to everyone who replied.

I went thru the same "System too rich Bank 1" ordeal on our 96 850. Turned out to be a vacuum leak plus a failing 02 sensor #1. The 02 sensors set a code of their own eventually but fail slowly over time so it may not be right away. Check out volvospeed.com/bay 13 for common vacuum leak locations. The ECT sensor is another common failure as the ECU "thinks" the engine is stone cold all the time and sets a rich mixture

bl

I have a 96 850 witht the same fault code (P0172). Car seems to be running fine, but I have to get an emmissions test and wonder if this will get in the way. What did you do to solve your problem? Thanks, Mark.

Just a week prior to the fault code showing up, I had the car in for inspection, which requires the emissions test. It passed with no problems, however, I might have just gotten lucky. The best thing you can do is what bl said and replace the O2 sensor and check for vaccuum leaks. I had the code erased, and after driving the car twice, the CEL came back on and reads the same code.

Forgot to mention, I believe the drive cycle madeinjapan mentioned is driving the car two to three times to normal operating temperature (at least that is what happened with my car after clearing the code). The code will set itself temporarily the first time you drive the car, or when the problem is first noticed by the computer and then the second time the problem is noticed by the computer, the CEL will come on. I think it does this to keep "false alarm" codes from being set.

Replaced to front O2 sensor and had two vacuum leaks fixed. One on the flame trap holder and the other in the rubber elbow down under the thermostat housing that you can hardly see. Are you seeing any black smoke or has your MPG slipped a little?

We don't have emission testing where I live (yet) but I believe this will cause you problems from what I've read here on the BB. Even if you clear the code & reset the MIL before going in, the test center can "read" the car's fault code history. Hopefully, someone who knows for sure will chime in.

bl

Yes, once the codes are read and cleared you have to go through a "Drive Cycle" to ready the car for the emissions testing. Do a search here or on www.swedespeed.com and you'll see what the drive cycle is.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.

A failing O2 sensor is a good bet especially if they have never been changed. Bank 1 is the sensor before the catalyic converter and Bank 2 is the one behind the CC.

bl

Thanks, i'll test both this weekend...