Ford Mass Air Flow Sensors

I get a lot of calls and e-mails asking about Ford Diagnostic Trouble Codes (DTC's) people pull out of their Powertrain Control Modules (PCM). Now you have to be very careful when you pull a code. Often the code refers to a symptom and not the actual problem. They can be a simple 'cause and effect' code. But that's something for another article. I want to talk about some specific codes on most Ford products.

Common Ford DTCs...

The codes I hear about most from Ford owners are P0171 or P0174; Fuel system lean, Bank 1 or 2. Now these codes indicate the engine is not getting enough fuel. This can be due to several factors, a bad Coolant Temperature Sensor (CTS), vacuum leaks or dirty injectors, among other things. One or more of the following symptoms accompanies these codes:

• Lack of Power.
• Spark Knock/Detonation.
• Bucking and/or Jerking.
• Hesitation or Surge on Acceleration.

Some other common codes are:

OBD-II Codes:

• P0172, P0175 (Fuel system rich, Bank 1 or 2)
• P1130, P1131, P1132 (HO2S11 lack of switching, Bank 1)
• P1150, P1151, P1152 (HO2S21 lack of switching, Bank 2)

OBD-I Codes:

• 181, 189 (Fuel system lean, Bank 1 or 2)
• 179, 188 (Fuel system rich, Bank 1 or 2)
• 171, 172, 173 (HO2S11 lack of switching, Bank 1)
• 175, 176, 177 (HO2S21 lack of switching, Bank 2)
• 184, 185 (MAF higher/lower than expected)
• 186, 187 (Injector pulse width higher/lower than expected)

What Causes These Codes To Set...?

Excellent question! As I said before there can be any number of causes. And I also said you have to be careful. If one sees a DTC P1130, P1131 or P1132 one might think there is a bad Heated Oxygen Sensor (HO2S) and you may be correct... but not this time. A lot of HO2S's are sold at about $100.00 that does not correct the problem. Normally an HO2S sends out a voltage signal from .1V to .9V, or lean to rich. If the PCM gets a lean signal from the HO2S it will richen the fuel mixture to compensate. And if it gets a rich signal, it will lean out the mixture. The computer is looking for the ideal voltage signal of .5V from the HO2S.

This never happens so the HO2S signals ranges, or switches, up and down around that .5V signal. Now when the mixture is so lean that the HO2S stays at the .1V, or the .9V, signal and the computer cannot lean or richen the mixture enough to compensate, you get the "Lack Of Switching" DTC. There is nothing wrong with the HO2S; in fact it's doing exactly what it's supposed to do. Replacing it will accomplish nothing, except confirm there is nothing wrong with the HO2S you replaced. When the PCM sees this lack of switching it throws this code and turns the Check Engine Light (MIL) on.

What Causes These Codes...?

Now we get to the crux of the problem. As I said these codes may have many different causes, but in almost every Ford Product from 1990 to 1999 the most likely cause is the Mass Air Flow (MAF) Sensor.

What Does The MAF Do...?

The MAF is mounted on the corner of the engine air cleaner. It uses a hot wire-sensing element to measure the amount of air entering the engine; air passing over the hot wire causes it to cool and sends out an analog voltage signal to the PCM to determine the intake air mass. The PCM will then calculate the required fuel injector pulse width to provide the desired air/fuel ratio.

Auto manufacturers have a cleaning cycle built into the MAF circuit. How it works is this; when the key is turned off a larger current is sent through this hot wire causing it to glow red hot, thus burning off any contaminants. Well, almost all auto manufacturers that is.

What Happens...?

MAF sensors can get contaminated from a variety of sources: dirt, oil, silicon, spider webs, potting compound from the sensor itself, etc. When a MAF sensor gets contaminated, it skews the transfer function such that the sensor over-estimates air flow at idle (causes the fuel system to go rich) and under-estimates air flow at high air flows (causes fuel system to go lean). This means Long Term Fuel Trims will learn lean (negative) corrections at idle and learn rich (positive) corrections at higher airflows.

Fixing The Problem...

To confirm this disconnect the MAF sensor connector. This puts the vehicle into Failure Mode and Effects Management (FMEM). In FMEM mode, airflow is inferred by using rpm and throttle position instead of reading the MAF sensor. (In addition, the BARO value is reset to a base/unlearned value.) If the lean driveability symptoms go away, the MAF sensor is probably contaminated and should be replaced. If the lean driveability symptoms do not go away the MAF sensor is probably not contaminated.

When this happens Ford says you must replace the MAF since they cannot be repaired. With a dealer cost of about $177.00 and about 1.5 hours of labor the repair is not very expensive. However Ford is not correct. This problem can be fixed and it can be done by just about anyone.

What you need to do is go to the local Radio Shack and get a can of spray tuner cleaner, #64-4315. Disconnect the NEGATIVE battery cable and remove the MAF from the vehicle. Spray the inside of the MAF with the tuner cleaner. Be very careful when you spray the hot wire. It is very thin and if it breaks, you will have to replace the MAF. Clean it well and let it air dry for 15 or 20 minutes.

Don't use carburetor or brake cleaner. They leave a residue and you will be right back where you started. Use ONLY the tuner cleaner, which dries residue free. When it's dry put it back in the vehicle and reconnect the NEGATIVE battery cable. You will need to drive the vehicle several miles to let the PCM "relearn" your specific driving conditions. Once that is done the codes will not come back and you should notice all the symptoms are gone as well.

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