QuestionI just bought a 2005 Sonata with a 2.7L V6. Everyother day it is getting a P0172 and P0175 DTC set (System 1 and 2 Lean). I have tried cleaning the MAF sensor and put in a new air filter element. I do not notice any leaks around the injectors. What could cause this? I have read that it could be an O2 sensor (not sure which one though if you could help with that), a pressure regulator or a leaking fuel injector. What about a clogged fuel return line? The problem seems to occur at low speeds (below 40 mph) and low RPMs and all DTC freeze data does not look too out of whack. PLEASE help with any advice you can lend. Thanks.
AnswerHi, Tom. I'm going to incorporate your comments that these codes represent a rich condition into this answer and delete the added information. The pertinent data in your freeze frame (and current) data should be the fuel trims.
The relevant oxygen sensors are the upstream sensors. Since you have a rich condition on both banks, however, it's not likely to be an oxygen sensor issue; if it were, both upstream sensors would have to be defective, and both would have to be defective in the same manner. That's just not a likely scenario. We're better off looking for a problem that would affect both banks.
If you have a scan tool capable of monitoring current data, attach it and monitor the data. If the problem is presently occurring, you fuel trims should be high positive numbers. Under normal conditions, the short term plus long term on each bank should add to no more than +/- 8%. For example, if the fuel trims on bank one are -10.0% short term and +5.0% long term, this would be normal (total -5.0%). If the trims are -10.0% short term and -5.0% long term, this would be too rich (total -15.0%).
First check to see if the purge control valve is stuck open. If you're looking at current data on the scanner, you can simply pinch the purge hose shut and monitor the fuel trims to see if they change accordingly. Otherwise, you'll need to check to see if you can blow/suck through the purge line (use caution-- fuel vapors will be in the line). If you can, the purge control valve is stuck open and should be replaced. You'll want to do this disturbing the purge control valve as little as possible, as it's common for stuck solenoids to begin working properly if they're vibrated.
If the purge valve is okay, then perhaps check your fuel pressure. It should be about 44PSI. If it's within a few pounds of this, fuel pressure is not the issue. If it's much higher than this, you'll need to investigate the cause, which at this point would be a blocked regulator or return line.
Last, I'd recommend checking the air flow sensor. It's very rare for the sensor to fail in such a way as to indicate more air than is going through the sensor, but that's still potentially possible. You should have about 0.5V (10kg/hr) at idle. Unfortunately, this is a rough number, so if you think this is the issue, you're pretty much left to try another sensor to see if the air flow or fuel trim values change. I still think this would be more likely than two failed oxygen sensors in exactly the same manner.
