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I would check the real temperature of the engine with a lazer pyrometer, that way you will know the actual engine temp to see if the coolant temp sensor is telling you the truth. If the coolant temp and engine temp agree and are too low, say around 150-160 degrees, then replace your thermostat so your can get your engine into closed loop. Until your engine goes into closed loop, the fuel trim not be active and its reading will not make any sense. Fuel trim is an adjustment that is made by the PCM to keep the engine fuel control within a tight spec window, so the engine never runs too lean or too rich. Some call this stoichiometric fuel control. The PCM will add or subtract from the millisecond injector 'on time' which adds to or subtracts from, the amount of fuel delivered to the engine.
P0125 indicates that the engine takes too long to warm up to the minimum closed loop operating temperature which is generally around 170 degrees F. This is almost always caused by a "lazy" thermostat that is stuck open or opens too easily. Replace the thermostat with one that is OEM spec, clear the codes and this will solve the P0125 issues. The P1155 is an air fuel ratio sensor heater problem on bank 2 sensor 1 (this is the cylinder bank that is opposite the bank that has the number 1 cylinder) check the resistance of the heater element in the sensor and if out of spec, replace the A/F sensor, however, please verify that you have a good 12 volt signal to the heater circuit at the proper time before you condemn the sensor. An inexpensive OBD-II scanner will help see this. Hope that this helps.
Best to call a Nissan dealership and have them run your VIN (17 digit vehicle identification number) to see if they have extended the warranty for the Fuel Level Sensor. This is because your vehicle is way past the typical emissions warranty of 3yr/50,000 miles. However if this problem is a known pattern failure of this year Xterra, then you may be covered. A vehicle that is approaching 10 model years old will have some components fail. Also, it's not that difficult to replace the sensor, Nissan usually builds in a panel that allows you easy access to the fuel tank sensor.
It is best to go to the manufacturer's technical web site and look under the technical sections for the translations of the hexadecimal values for your specific vehicle. Dodge may charge you for this info, I don't have it, as I would have to pay to log in as well. If your car passed all the monitors, then you are good to go. If your idle air valve is defective and your are sure of that, then by all means, replace it. As for your fuel economy, look at your fuel trim. Is it rich or lean? When you say bad fuel economy, could you be more specific? How many miles on your vehicle? I would check to make sure your timing chain is not too stretched, as retarded timing will kill your mileage BIG TIME! Do a base line timing inspection. It can be done even though your 5.9 has its timing controlled by the computer. Feel free to ask more questions. Dan
Hi, my name is Dan. It would be nice to know what engine you have. It would be great to know what the fuel trim is, just so you know whether the engine is running lean. I mention this because you say that the misfire gets worse as the engine warms up. As the engine warms up, the fuel mixture goes leaner, because a cold engine needs a richer air/fuel mixture. If the long term fuel trim is say 11-14%, that can cause a rough idle and may not set a lean code. You say the rough idle never sets any misfire codes, have you sprayed carb cleaner or used propane to see if you have any vacuum leaks i.e intake manifold/plenum gasket etc ? Does the EGR valve stay fully seated? I test for this by raising the valve and then SLOWLY closing it to see if there is any leakage. Then I raise it and snap it closed by suddenly pulling the vacuum line off. If the idle is different when snapped shut, you have discovered something real. Also, check for any oil on the ends of the spark plug boots, as I have repaired many misfires due to the spark plug wells leaking oil on the spark plug boots. The oil acts as a conductor and the spark voltage bypasses going to the plug and goes directly to the head. I assume you are using OEM plugs and wires i.e genuine Ford/Motorcraft. This really helps as I have chased so many issues with aftermarket parts, so keep this in mind. Also, a fuel system cleaning can really help, I always do this whenever I repair a misfire in the shop where I work. I have a machine to do this. You can use Chevron Textron, just put a bottle ( as per the instructions )in your tank. I would use it for at lest 2 full tanks, Mercedes uses it on all of their vehicles when they come in for a service, that speaks volumes. To check long term fuel trim ( lean or rich compensation. a high positive number i.e. 15% means the power train computer is adding 15% more fuel than normal etc ) you can get an Autel CAN/OBD II code reader at AutoZone. ($50 ? or so) It has a data stream feature that shows long term fuel trim, I use one on test drives to set the OBD-II monitors, because I don't want to lug around the Modis or another heavy, full featured scan tool. Let me know if any of these suggestions shed some light, I will help you any way I can. Sincerely, Dan