Engine check light displayed

SOURCE: 2002 X-type Jaguar with 2.6 -- check engine light came on

My 2002 x-type did the exact same thing on many occasions and had many repeated trips to the dealership. This problem was ultimately cased by cracked hoses in the car's vacuum system (which is used in the cruise control and brakes). Have the dealer check the vacuum lines and the breather hose (mine had completely collapsed). Chances are they are leaking and causing your car to "buck" and the cruise to not work. Don't ignore this as it will affect the functionality of your brakes too over the longer term.

SOURCE: p0171 and p0174 codes

P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)

If a car has a check engine light on and has P0171 and P0174 codes stored, the most likely cause is a vacuum leak. These codes are set by the car’s computer when it sees too much oxygen in the exhaust. Excessive oxygen readings in the exhaust will set lean codes. This does not mean that the oxygen sensors are defective. It is a common mistake for people to replace oxygen sensors when lean codes are present. Just because the code is related to oxygen sensor readings, it doesn’t mean that the oxygen sensors are bad. If both of these codes are stored this means that both banks of the engine have excessive oxygen in the exhaust. If the oxygen sensors are replaced it’s most likely that the same codes will reappear. It is very uncommon for two oxygen sensors to fail at the same time.

Therefore the most likely cause of P0171 and P0174 codes is a vacuum leak. The source of a vacuum leak can be one of many things. The most common cause of a vacuum leak is a leaky hose. Vacuum hoses over time can become dry and brittle. The rubber deteriorates and can leak causing a vacuum loss. Many times a leaky vacuum hose can be found by listening for a hissing sound. A large vacuum leak can even make the car run rough at idle. In some cases it will not idle without feathering the gas pedal.

SOURCE: check engine light codes p0171 and p0174. What do

these are lean codes. 171 is system too lean bank 1 and 174 is system too lean bank 2. what this relates to is you probably have a vacuum leak. look under for signs of obvious breaks in vacuum hoses. this is your most likely culprit. happy holidays

SOURCE: Jaguar x type 2004 2.5 petrol V6, 38500 miles, codes P0171, P0174

Have exactly the same issue on 04 jag x type 2.5. So far have replaced fuel filter, air filter k&n, plugs ngk. Codes cleared, same issue. Seems to appear when air temp is cold and eng is warm.
Am replacing upstream O2 sensors and cleaning MAF next. Will keep you posted if it clears up. Did you ever find the cause?

SOURCE: 1996 jaguar xj6 dtc test code p1178

Fault code P1178Rich Fuelling Trim, Long Term (AMFR)Check for manifold air leaks, rectify as necessary Check for recorded MAFS fault code, rectify as necessary Clear fault code and perform service drive cycle to verify fault cleared Contact Jaguar Service Hotline if fault still present.

Taken from the Jag service manual.