OBD Explained

Too much air and/or too little fuel to Bank 1 (presumably a bank of engine cylinders)?

The code will determine the bank, the breakout will determine how many banks and which cylinders are included in aforementioned banks. I assume you know what lean means.

First thing I would check is the oxygen sensor. An oxygen sensor going south can cause the engine thinking it is running lean. This problem very seldom shows up in the explanation of the diagnostic code. If the engine run smooth, this is the first thing I would change. I believe that the code should show up for both sides if the MAF sensor was dirty, but the sensor is very easy to clean.

Is it easy to swap the sensors in bank 1 and bank 2? If it is then try it: if the fault "follows" the sensor (i.e. you get a P0174), then it's the sensor that's duff.

I got this code on a Jeep Cherokee Country. It is a straight line 6 cylinder and the "left bank lean made absolutely no sense to anyone I talked with. The Haynes repair manuals and many google searches did not adequately explain it either.

After about a year of cleaning injectors and replacing cheaper parts, the gas milage suddenly went down 8 mpg.

I bit the bullet and replaced both upper and downstream oxygen sensors. They were caked with carbon. (I thought there was a specific code that came up when they went bad).

The Jeep now runs much better, the gas milage is back to 20 mpg, and the code has not reappeared.

Funny thing. When I had an exhaust manifold leak, I would get a catalytic converter efficiency error from my code reader (P0420). This was one of those counter-intuitive situations.

As it turns out, when you have an exhaust manifold leak between the head and the manifold two things happen. When the exhaust valve opens and the pressure is high, exhaust leaks out and after the exhaust valve closes, the high velocity gases pull excess air back INTO the exhaust manifold. This might explain a lean system code.

This excess air reaching the catalytic converter confuses the upstream and downstream O2 sensors, and the on-board computer thinks there is a problem with the "oxygen storage" of the catalytic converter and it trips a trouble code.

This site actually mentions about the exhaust leaks:

http://www.obd-codes.com/p0420

Many other sites with help tips for OBDII codes were not as complete.

I'm glad I didn't run out and drop $900 on a new cat for my 'Burb when the problem was really a small exhaust leak. I did start by replacing the O2 sensors but those were only about $60 each and they did have over 150,000 miles on them.

More on the O2 sensor and cats (not Del) from these great folks here:

http://www.magnaflow.com/07techtips/techbulletins.asp

http://www.magnaflow.com/07techtips/TechBulletin/TB80009.pdf

http://www.magnaflow.com/07techtips/TechBulletin/TB80020.pdf

http://www.magnaflow.com/07techtips/TechBulletin/TB80003.pdf

They write nice, clearly written articles about various exhaust topics easily understood by the novice, yet informative to the serious enthusiast. Disclaimer: I don't have any financial, familial, or emotional interest in the company, but when I need replacement parts for my exhaust system, I will probably go to them first.

Good luck with your quest.