Vehicle Mass Air Flow (MAF) Sensor Failure Research

There are several kinds of MAFs on the market these days, the two most common being hot-wires and thin films. Both are a version of Wheatstone Bridge technology and operate in similar ways. Others, such as vortex shedders and vane meters, are not as common in today's cars and trucks.

Usually MAFs are built very reliably and are quite robust but in the late 1990s there were major manufacturing failures by several MAF makers. Over one million vehicles were recalled by the Big 3 and some foreign car makers.

Some aftermarket vendors got caught in the crossfire and were blamed for multiple symptoms that went far beyond the MAF itself. The issue we researched was the claim that the oil from oiled air filters damaged the downstream sensors. The task at hand was to determine whether or not the sensors being returned for claims had failed and, if they did, why.

Every MAF submitted underwent several tests at K&N Engineering. A summary of the most recent tests can be found here.

We quickly discovered that sensors returned had widely varying issues. Some showed evidence of contamination on the sensor elements themselves, some had electronic circuit problems, but the majority of the sensors tested were not only functional, they were within the manufacturer's specified accuracy limits.

The EPA is mandated to regulate automotive emissions under the authority of the federal Clean Air Act (CAA). Section 207 of the CAA gives the EPA the mandate to require a manufacturer to recall and fix vehicles where a substantial number of any given make and model or vehicle class do not meet the current emission standards for that model year. The EPA maintains certified testing laboratories to check and enforce compliance.

The EPA has been publishing recall data since the CAA was amended in 1990 mandating rules for On-Board Diagnostics (OBD) and inspection/maintenance programs (I/M). These recalls are based on tests performed on randomly chosen, as-manufactured vehicles at the certified labs.

Aside from the recall failures due to manufacturing defects, MAFs are susceptible to the harsh environments under the hood and suffer most notably from the 'chimney effect'.

When an engine is turned off it's hot and there are a number of hydrocarbon compounds, in vapor form, residing in the intake and exhaust manifolds, cylinders, and crankcase. These vapors eventually condense as the engine cools and they do condense on the sensor elements and exposed thin films of the MAF sensors. Unfortunately, this leads to an early MAF demise.


One response to this problem was the subtle installation of activated carbon traps in the intake tract of late model vehicles. Effectively they act as sponges for volatile vapors and minimize the effect on delicate sensors in the tract.

Another problem specific to the thin film sensors was delamination. Sensor material is carefully bonded to a supporting substrate. Early on, the temperature cycling and vibration were enough to cause a critical portion of the sensor element to delaminate from the substrate and break the circuit.

These were only two of the failure modes we uncovered during the course of the project and involved researching hundreds of failed MAF sensors from all major brands and vehicle makes. It was an incredible amount of fun.



And here's a tip - the quickest way to destroy your MAF (and possibly your engine) is to run without an air filter.



Don't do this to get more airflow.