Mass Air Flow Sensor Testing,P0171-P0174 System Lean
Vehicle: 2001 Land Rover Discover II Codes: P0171, P0174, TCM Incorrect Gear Ratio
Symptoms: NONE, Slight pre-detonation (pinging) when hot
STRAIGHT TO THE POINT:
The vehicle has no drivability concerns and was hard to diagnosis. The TCM (transmission control module) is setting incorrect gear ration codes which means the transmission is slipping (usually). Then of course P0171 and P0174, lean both banks. Fuel pressure was within specifications (45-50 psi) and the MAF was reading correctly, I thought! I can hook up the scope and perform a snap throttle test. The signal voltage reads anywhere from 3.7-4.0 volts which is ok from my experience. This is why I’m writing this article. The best way to diagnosis these slight differences in MAF signal is with a scanner that has a good graph display. Take a look at the MAF signal graph display below. The MAF signal that’s causing this code to set does not have an obvious problem, the signal looks ok at a glance. The part about the incorrect gear ration codes setting is only related to Land Rover’s. It has something to do with the way the on-board computers operate. A weak MAF signal should not set a incorrect gear ration code, but it’s a very good thing to take note of in case you ever run into something like this again.
Another good way to monitor this weak MAF signal is through the short term fuel trim. The signal are pretty much in correlation with each other. Fuel trim is a very important data value to monitor and understand when diagnosing automotive drivability problems. For example take a look at the before and after pictures of the fuel trim graph display above. If you work on these problem enough it’s a good idea to snapshot these type of value displays for quick reference later on. When you do eventually run into a similar problem it will save a lot of time, but the key word is quick reference. If it takes you too long to pull up these snapshots then it defeats its purpose.
Short term fuel trim is also very good for diagnosing vacuum leaks. Monitor your fuel trim at idle, if you engine is running lean your fuel trim is high. Raise engine to 2500 RPM’s, hold and re-check. If the fuel trim comes down chances are you have a vacuum leak. The reason for this is your vacuum is lower at off idle and therefore less un-metered air is getting in to the cylinders. If the lean condition was caused by low fuel pressure, MAF/MAP sensor, A/F sensor, ect……. The fuel trim would not decrease when at 2500 RPM’s. Most likely the fuel trim would increase depending on the actual problem present.
DIAGNOSTICS FlOWCHART P0171-P0174:
A code P0174 may mean that one or more of the following has happened:
•The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of “oiled” air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
•There could be a vacuum leak downstream of the MAF sensor.
Possible solutions include:
•In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for its location if you need help. I find it’s best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it’s dry before reinstalling
•Inspect all vacuum and PCV hoses, replace if necessary
•Check for a dirty fuel filter and proper fuel pressure
System Too Lean (Bank 1)
What does that mean? Basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. The P0171 is one of the more common trouble codes. This code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine’s cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code. You’ll want to also read our article on short and long term fuel trims to help understand the operation of the engine. Note: This DTC is very similar to P0174, and in fact your vehicle may show both codes at the same time.
You will more than likely not notice any drivability problems, although there may be symptoms such as:
• a lack of power
•detonation (spark knock)
•hesitation/surge on acceleration.
A code P0171 may mean that one or more of the following has happened:
•The MAF (Mass Air Flow) Sensor is dirty or faulty
Note: The use of “oiled” air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
•There could be a vacuum leak downstream of the MAF sensor
•Possible cracked vacuum or PCV line/connection
•Faulty or stuck open PCV valve
•Failed or faulty oxygen sensor (bank 1, sensor 1)
•Sticking/plugged or failed fuel injector
•Low fuel pressure (possible plugged/dirty fuel filter!)
•Exhaust leak between engine and first oxygen sensor
A lot of times, cleaning the MAF sensor and finding/fixing vacuum leaks fix the problem. If you’re on a tight budget, start there, but that may not be the fix for certain. So, possible solutions include:
•Clean the MAF sensor. Consult your service manual for its location if you need help. I find it’s best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it’s dry before reinstalling
•Inspect all vacuum and PCV hoses, replace/repair as required
•Inspect all hoses and connections in the air intake system
•Inspect and/or test the intake manifold gaskets for leakage
•Check for a dirty fuel filter and proper fuel pressure
•Ideally you’ll want to monitor short and long term fuel trims using an advanced scan tool
•If you have access, you may want to run a smoke test
Transmission Codes are set from faulty MAF Sensor for the Land Rover – TCC Lockup and incorrect gear ration codes so if you have transmission codes ck the MAF Sensor. I have even seen it where there is no drivability concerns at all and you scope the sensor and it check fine this is where you have to dig deep into the diagnosis and I have shown you how.
MASS AIR FLOW SENSOR OPERATION:
Check engine light faults relating to the Mass Air Flow Sensor are becoming less common, but do occur. The tricky thing with some of these types of faults is that you can have a mass air flow sensor concern without triggering the check engine light.
Before we get into diagnosis let’s start with a brief overview of the sensor itself. The main purpose of the Mass Air Flow (MAF) Sensor is to measure the volume and density of air entering the engine at any given time. The computer uses this information in conjunction with input from other sensors, to calculate the correct amount of fuel to deliver to the engine. Input from this sensor is also used indirectly to help calculate desired ignition timing and transmission operating strategies. MAF sensors are mainly designed as a “hot wire” sensor or a “hot film” sensor. Both sensors function in a similar fashion. Hot wire sensors pass current across a platinum wire and hot film sensors pass current across a foil grid. The current level is regulated to maintain the hot wire, or film, at a predetermined temperature. This temp is either a direct value, or a value that is a set number of degrees above ambient (outside) air temperature.
So how does this tell us how much air is getting into the engine? Well, as air passes across the mass air flow sensor, it cools the hot wire, increasing the amount of current needed to keep that wire at the specified temperature. The amount that the wire is cooled is directly proportional to the temperature, density and humidity of the air passing through the sensor and as such, the current increase needed to heat the wire allows the computer to easily calculate the volume of air entering the engine.
Mass air flow sensors typically either send a voltage, or frequency signal to the powertrain control module (PCM). Hot wire sensors typically have an operating range of 0 – 5 volts, with idle voltage being around .5 – .8 volts and full throttle application is normally between 4 and 5 volts. Hot film sensors typically produce a frequency output of 30 – 50 Hz with 30 Hz being idle and 150 Hz at full throttle. There are some other subtle differences between the sensors but these do not affect function or purpose.
So what types of symptoms can we get from MAF sensors, and how should we test for these faults? Well as we stated earlier, MAF sensors can produce drivability symptoms without generating a check engine light code, so some specific checks are in order. For ease of diagnosis a scan tool should be used to monitor sensor readings. In some instances it’s okay to take sensor value readings by back probing the appropriate terminals at the MAF sensor. If specific MAF check engine light codes are present, then proceed with the appropriated tests. If no codes are present, or if you have lean codes you suspect are caused by a faulty mass air flow sensor, perform the following. Obtain sensor specifications from a reliable source; you can send us an e-mail from the Help Link and we can assist with most information. Hook up a scan tool with the ability to monitor sensor valves (PIDS) and pull up the mass air flow sensor. Record your MAF sensor reading at idle, and again at various RPM ranges. Compare the values against specifications. Next, start from idle and increase throttle opening while watching MAF reading. Increase should be steadily proportional to RPM change. Perform the same checks while lightly tapping on the sensor, or heating the sensor with a blow dryer. Any fluctuation, or out of specification reading indicates a mass air flow sensor, or related wiring concern. Repair and retest. I would also recommend monitoring MAF values while driving the vehicle and checking readings while the concern is present. Have an assistant drive while checking these readings. If the mass air flow reading is within specification while a concern comes and goes then it is not likely the problem. Be sure to check all air intake connections and gaskets, as well as the air filter before faulting the sensor, as these types of concerns can affect readings.
it is not always necessary to replace a mass air flow sensor that is reading out of specification, though most dealers will tell you differently! It is possible that the sensor is just contaminated from age or use of oil saturated, aftermarket air filters. You could try exposing the sensor hot wire (once the sensor is removed from the vehicle) and cleaning it with electronic parts cleaner and low pressure air. Use appropriate cautions. Once the sensor is clean, reassemble and install and check operation, you may be pleasantly surprised! I hope this information has been helpful. Thanks for visiting , and have a great day!