A friend of mine picked up a 97 jeep wrangler with a 2.5 for dirt cheap. He started having problems with this code. On inspection of things he noticed that his cat was plugged. After cat swap we are still having issues. The scanner says this code is a lean code, when cross referencing this code to the manual it says that engine management is unable to lean the engine? The engine is running temp is at 100 and doesn't throw a code until it goes to open loop. There is a noticable miss during closed loop. Fuel pressure before the Regulator is a steady 50 psi. We swapped the regulator between a known good running jeep and no difference. Every sensor has been swapped by him. Also computers. Any help with this is greatly appreciated.  

A cracked or leaky exhaust manifold will cause an o2 sensor to read lean.

A dirty or worn injector will also cause a lean condition. Test the injectors with an ohm meter to see if the injector coils are within range. It's more difficult to see if they are flowing correctly, but a mis and a lean condition seems to point to injector issues.

A mis and a rich condition would indicate an ignition problem (wires, plugs, cap, rotor).

If the engine really is running at 100 degrees, it will never go into closed loop. The computer will also throw a DTC for taking too long to reach operating tempature. There is a sensor for the temp gauge and a temp sensor for the PCM input.  

I only stated the temperature to point out that it was missing on the computers set parameters while at closed loop. Engine heats properly. Your thought on the injector is the same as mine. With the 137,000 on the clock I wouldn't be suprised. I would like to know if anyone has an exact definition of this code that may be different than mine. He is faster to swap than I am. Im cheap plus I enjoy the frusteration when its on some one elses vehicle. Thank you very much for your help on this. Oh O2 readings were 3.04 upstream and downstream was 1.2 or so I believe. Within range.  

The Factory Service Manual says:

"A lean air/fuel mixture has been indicated by an
abnormally rich correction factor."  

I'm not sure of your o2 sensor readings. They don't seem to be inline. .1 volts would be lean and .9 volts would be rich. Normal 14.7:1 air/fuel ratio would be around .45 volts.

Here is a document that I like to refer to:

Click here for more info.  

Here is another just on o2 sensors.

Click here for more info.  

Thankyou very much. I believe your on to it with the lean readings. Any how he has replaced everything that would relate to timing and has believed it was a rich code. Thanks Ill let you know how it goes.  

Very good sites that shine light into the functions of the o2 sensors. I have learned alot from reading them and I thought that I had a good understanding of them. Even with the internet information like that is usually paid for. Thank you very much.  

I have a 1997 jeep Grand Cherokee, and i got an Engine code p0171 (system too lean bank 1). Although i have this code, it is dumping massive amounts of fuel in the system. I have changed the PCM, and have tried a new MAP sensor, there is no MAF sensor on it. The injectors are spraying fine and none of them are stuck open. Recently changed the fuel pump, o2 sensors and have a new catalytic converter. In 2 miles, the plugs are black, black sut and gas are on the ground from the exhaust. Im exhausted can you help? -Danny  

Where we are on our pos jeep. We have replaced every electrical ignition part on the jeep to no avail. We have a very nice code scanner and the sensors are reading outside of correct parameters even after replacement. We have checked every circuit. We replaced the PCM's. Im going to check the exhaust valves for bends or warps. Ill remember to keep updating this until we find the culprit.  

Well after pulling the head off the motor it was found to have severe pitting on the valves of 2 cylinders. After replacing the valves and refinishing the head the cat and the first o2 sensor needed replaced. No codes now. It runs like a top.