Hyundai Repair: Error Codes for 2001 Hyundai XG300, 2001 hyundai xg300, air flow sensor

Question
QUESTION: To whom it may concern,
My parents have a 2001 Hyundai XG300 which is getting error codes 171 and 1159. I'm sure you'll laugh but I went to Advance Auto Parts and they gave me the codes but their equipment has limited diagnostic ability. One is apparently about running lean and the other I'm not sure. The car was running fine and they cleared the codes. But obviously if it's a legitimate problem they're going to come back. I'm trying to see if it's something I could repair for them, since every time they bring this car in for service it costs them an arm and a leg. I would really appreciate some assistance with this matter.

I thank you in advance!

Matt

ANSWER: P0171 indicates a lean condition.  The most common cause of this on the XG is a defective air flow sensor.  To do a proper diagnosis, you'd need a scan tool capable of reading the stored data (freeze frame) at the moment of failure and the data stream from the vehicle.  We already know that at the time of this failure the bank 1 fuel trim* was very large; that's what causes this code to set.  What we don't know is what the bank 2 fuel trim was at the time of the code setting and what both banks' fuel trims are now.  It's important to know the bank 2 fuel trim since this will tell us if the problem is something that affects both banks somewhat equally.  Often, one bank's fuel trim will be just over the threshold for setting the code, while the other bank's fuel trim will be just under.  It's important to know the current data as this will tell you whether the problem is still occurring.  Once we know these things, we can eliminate potential causes that aren't consistent with the data.  For example, if one bank reads normally while the other reads high enough to set P0171, then we would expect an issue that would affect only that bank, such as the oxygen sensor.  If both read similarly, expect the issue to be the air flow sensor or other device that would affect both banks somewhat equally.

P1159 indicates the variable intake valve does not reach its desired position.  Expect to need to replace the intake plenum to resolve this.

* Fuel trim represents the amount of time the fuel injector opens for each intake cycle, expressed as a percentage above the expected amount of open time based on the data from the oxygen sensors, air flow sensor, and throttle position sensor.

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QUESTION: Thank you HT, the info was informative and helpful. I would like to ask what issues might cause the plenum to need to be replaced? Although the car is pretty old my parents don't travel very far and the car has very low mileage. Just curious what might cause such a repair to be needed. I believe it would be quite expensive to replace the plenum?

I understand that unfortunately I would need more information to accurately diagnose these problems. Not sure where I'll go or what I'll do from here. The dealer may be the only option?

Thanks again,
Matt

Answer
The plenum has on it an actuator which opens and closes plates inside the plenum to effectively change the manifold runner length.  The P1159 code means that the actuator cannot move the plates to the desired position.  Either the actuator has failed or the plates have stuck in place.  In the former case, repair is probably impossible.  In the latter, the potential is there that you could disassemble the plenum and free up the components when you discover what it is that is keeping them from moving properly.  In either case, the individual parts of the plenum aren't available separately, so if the issue requires a part replacement, you'll need to replace the entire plenum.

If you're looking at having a shop diagnose and work on the car, choose a shop with an impeccable reputation for being very good with diagnostics.  This may or may not include the local dealer.  The P1159 code isn't terribly difficult to diagnose; this is evidenced by the fact that I can pretty much tell you now that you'll need a plenum.  The P0171, on the other hand, has a host of potential causes; this is where diagnostic capabilities are invaluable.  Choose the wrong shop, and this could turn into an expensive series of guesses.