2009 Impala 3.9L P0172 and P0300

The driver of this cab of ours said car was chugging and having trouble on hills. I pulled these codes from it and monitored the current misfire pids and there wasn't any. There was plenty in the history though! I pulled up the STFT and LTFT pids and noticed that LT was sitting at 10%. The voltage on the downstream O2 was steady at 700mV while upstream seemed to be working fine and had close to 0 FT numbers and good voltages. I tested the signal circuit integrity of the downstream by using test light connected to ground and touched the signal side of disconnected harness..and it was good, it went to 0 from the 454mV (KOEO). The heater circuit of the downstream also seemed ok at 5 ohms.

Since the plug wires were an aftermarket brand (we just got this car a few months ago and at that time had not put new wires on since was running good at the time)I decided to go ahead and put on OEM plugs and wires. The air filter was slightly dirty, so I put in a new one, thinking that may be contributing to the rich readings. But it did not have any effects on what the FTs where showing.

Car is running good at the moment but that 10% LTFT is bugging me...what could be causing it. I even reset the learned FT while watching the FT pids and it reset to 0 and then immediately adjusted right back. So I guess that at least shows that the O2s are working right.

Just not sure on what direction to look to find why its running slightly rich, for I don't imagine the cat will like dealing with the extra HCs all the time!

do you mean the trim number is -10?? because 10 is a lean condition not a rich.... what are the trim numbers on both banks at idle and 2500 rpm?

Sorry, yes its -10 on downstream. At home now so can't get any more info atm from it.

id do an injector balance test on that bank to start out with. a bad maf would effect both banks and so would fuel pressure.

Oooooh, sounds like a fun one! For what it's worth, I wouldn't sweat the -10% trim right now. See if you can get it to either misfire, or run rich enough to code, and go from there.

Maybe check freeze frame data? I'm thinking of an upstream O2 sticking high, causing a false rich condition, which would take all the fuel away (chugging, low power, misfires) AND cause the codes you saw. You could confirm this by comparing the upstream to the downstream and see if they agree. I'm thinking of this video in particular:



Also, what's the fuel composition %? Wondering if the P0172 is due to an incorrect alcohol content measurement, and the misfire is a different problem entirely.

wouldn't the fuel composition sensor cause both banks to run rich or lean depending on inaccuracy? not just one bank?

I don't think these had bank-to-bank fuel control. You'd expect an '09 to have it, right? :blink:

I looked at the wiring diagram for this vehicle and it only has one upstream and one downstream, so whatever was causing your p0172 could be effecting both banks. I would definitely looks at resetting the Fuel composition and maybe checking your purge valve for a leak.

I don't believe either of those would cause a misfire.