Codes P0171 & P0174 (Too lean bank 1 & Too lean bank 2)


Asked by MtnHiker Jan 31, 2013 at 08:20 AM about the 2002 Jaguar X-TYPE 2.5

Question type: Maintenance & Repair

I have checked all vacuum lines, sprayed carb cleaner around MAP sensor, Intake manifold gaskets, between throttle body and manifold and around all vacuum points. All are tight. I've cleaned the MAF sensor and throttle body. Ive read that fuel filter might need changed, but 95% of posts for Too lean on both banks point to vacuum leak. Anything I'm missing here or should I go for the fuel filter next?

Just as a little history - My employees car. She drove it to failure last year due to overheating. I replaced her waterpump and thermostat last year and it ran well. Started having issues this winter and she didnt tell me till it alkmost wouldnt move. It was throwing over a dozen codes. Just replaced all sparkplugs and one ignition coil and oil change etc. Also found the front valve cover to be loose and fixed that. Runs like a champ now, but has those two codes still. Am Guessing the fuel filter does need changed as she hasnt maintained well and it does have 109,000 on it, but really want to find the code issue.

11 Answers

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Well, you seem to be doing all the right things ...but too lean on both banks could be the MAP is faulty and cleaning it didn't do the trick. Replace it..easy for me to say I know they are a hundred bucks..but maybe it can be tested before you put out that much for a 'maybe'

8 out of 8 people think this is helpful.
Tom Demyan

Codes P0171 & P0174 usually means unmetered air is getting into the engine. Not detected by the MAF. You said you already cleaned it, but a vacuum leak is always the culprit when it comes to these codes. Could be a faulty hose, or an intake gasket. Some mechanics will "smoke" the car to find the leak. The way I usually find them is start the car, and get under the hood and start moving the hoses around and you'll here a hiss. Also look for collapsed hoses and loose hose clamps.

4 out of 4 people think this is helpful.

If there are no intake leaks, the next step is to replaced the air filter and clean the air flow meter. If the problem persist the front oxygen (O2) sensor may need to be replaced. --- Here's a link to Jaguar specific codes - there are some TSB links at the bottom. ---

12 out of 12 people think this is helpful.

Considering the repair history and mileage, and that it is a consistant hard code, it could be the O2 sensors are fouled and not responding quick enough. I worked on a 1981 Cadillac 6-8-4 engine where the check engine light would flicker while driving but wouldn't set codes. Most notable symptom was that while driving with the light flickering you could use the AC control head (used to control diagnostics) to force 4, 6, or 8 cylinder operation without going into diagnostic mode. I found out, several years later from a Cadillac Service Engineer, that the sluggish O2 sensor was putting the computer into a factory production line diagnostic test mode(negative voltage on the sensor input accessed by the worker from a green connector under the hood). Checking fuel pressure will give you an idea about the fuel filter or imminent failure of the fuel pump. Clogged fuel tank venting could also cause a reduced fuel pressure. A couple of fouled injectors could also be causing it to run lean. In 1984 I ran into a whole slew of Oldsmobiles that consistantly fouled injectors when using Texaco gas. I think a clogged air filter would give a rich condition.

2 out of 2 people think this is helpful.

Hi guys let me share you my experience in here with my jaguar x type 2009 ,what you call it limp mode or engine hesitation or engine stall on 3000 RPM, initially the problem was intermittent then became hard,and this was my troubleshooting on that 1- fault codes pulled and showing both the codes P0171 and P0174 both banks lean 2- unable to observe any air leak related to hoses between brake booster and inlet manifold and vacuum pump, i can hear hissing sound but unable to locate that while hoses in place and due to lack of proper tools 3- i start thinking to prepare my self to put all the possibilities to gather such as booster hose ,fuel filter,inlet manifold upper and lower gaskets and fuel injectors 4- starting with the booster hose , i removed the hose and gues what ?????? i found the check valve totally deteriorated and heavily leaking but i never had a brakes issue, same replaced thinking that the issue killed but unfortunately i did not erase the fault codes and after drive test no change the problem still persist here i need to tell you may be i did a mistake as i did not clear the fault codes 5- air inlet manifold removed and both upper and lower gaskets replaced , fuel injectors cleaned then after all of that the problem still persist after the test drive and even became more worse, i really got disappointed then it click to me to erase the fault codes ...... woooowoo ...... ya ... the problem cleared and the engine running and accelerate like wonderful. No fuel filter ,MAF sensor and O2 sensors not touched i think the problem solved once i did replace the brake booster hose but the problem persist as i did not clear the fault codes , i was looking for the check valve to show the valve status but unfortunately it looks i but in the Bin

6 out of 6 people think this is helpful.

There are two actuators (black & round) on the left side of the intake manifold that control the blending of short runner/long runner. The have round green gaskets under them that are known to leak and cause stumbling/rough idle, mixture problems, hissing, unmetered intake leakage. On the 2.5 the one closet to the rear can be changed in place by tilting it toward the front of the vehicle to remove and reinstall it. The 3.0 often requires removing the intake to replace it's gaskets for those actuators. I bought the gaskets for my 2.5 but have not yet needed to replace them. They are not expensive.

1 out of 1 people think this is helpful.
Brent Herling

My 2000 XKR suddenly developed a rough idle. Engine codes showed too leak bank 1 and 2. Shots of throttle cleaner identified the area to the rear of the intake. I found a bolt missing on ERG valve. Problem solved.


I had the same issue. Driving me crazy because I was trying to sell my 2005 Jag X-Type AWD.... I looked under the hood and found that my air cleaner was not screwed down..... I screwed the air cleaner back down reset the codes and now no more issues.... I lost a sale yesterday because of that...

1 out of 1 people think this is helpful.

The air cleaner box may have not the problem as the light came back on and same codes. I started looking for an air leak and found one. See attached picture. The bushing that holds the vac tube in on the upper intake was slightly cracked. When sprayed with carb cleaner the engine would hesitate. I used high temp rv silicon and so far the light has stayed off.

4 out of 4 people think this is helpful.
Deshannon McCrary

I wanted to say thanks guys. I have a 2005 Jaguar X-type. I had the same faults P0171 and P0174 and changed out the O-Ring, Air filter, Fuel filter, cleaned the MAF sensor and still no change. I then decided to change out the brake booster hose and sure enough, it had a nice size whole in the check valve (middle part). I replaced it and I'm waiting to see if the code clears. I was told it should clear on its on after I drive it around a couple of miles. Computer should reset its self. If it doesn't how do I clear the fault? I'm thinking the whole was pretty big and I don't think the MAF sensor needs to be replaced.

4 out of 4 people think this is helpful.

Solved my lean codes by chance when I installed (and sealed up this massive leak) a new flex pipe. Nobody thinks about leaks AFTER the combustion, but it is still a leak BEFORE the sensor, and therefore it affects the readings and will also give faulty cat codes. So check your exhaust up to the cats for any leaks before you tear down the intake and good luck.

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