Important article: http://mike.focke.googlepages.com/checkenginelightcausedbymassairflowsensor The MAF sits inside the air intake just after the air cleaner, measures the air flow (mass and temperature) coming into the engine, and sends signals with the results to the engine computer so it can compute the correct amount of fuel to add to create the right fuel/air mixture. It is safe to drive the car while these Subcodes are present. But not very efficient and the engine probably will run rough and lack power because the computer isn't getting a signal that it needs to do its job. Long term, you can damage the expensive catalytic converters by dumping unburned gas into them as the engine's computer tries to compensate for what is really wrong. The MAF looks like this and is shaped so that it can only be inserted one way for the screw holes to line up. The sensors are the piece above that looks like half a paper clip and a shiny piece inside the tunnel just to the right of the paperclip in this picture. If you want to know more about how it works, look at http://autotechrepair.suite101.com/article.cfm/mass_air_flow_sensors (but you don’t need to know just to work on it). I’ve added what the repair manual says at the end of this page for background reading so you can see what Porsche says about the MAF. When the Engine Computer senses an individual signal or combination of signals that is not right, it sets the Check Engine Light (second from the left of the warning lights below your instruments) and the Secondary Codes so that a mechanic can see what is going wrong. These codes are present until they are cleared by an OBC II reader or until the battery is disconnected long enough to reset all the codes. MAF failure is a common problem in all cars and most often occurs because enough air has passed over the MAF’s sensors to render one or both of them dirty. Less often, the MAF itself electronically fails. MAF problems are especially common in cars with aftermarket air filters especially ones that are oiled (too much). To see if it is the MAF itself that is the problem, disconnect the MAF wiring harness and drive the car for a bit. Yes the car will drive without it because the ECU (Engine Control Unit) will go into a mode that doesn't use the MAF's input when it senses the MAF isn't sending any. If the problem changes for the better, either clean or replace the MAF. If it doesn't, you have another problem. Don't run the MAF-less car too long but a half hour won't hurt and is long enough for your diagnosis. To disconnect the MAF wiring harness, follow the first part of the "Cleaning" instructions below for raising the engine cover and remove the wires from the MAF without removing the MAF from the air intake. Should take no special tools and only about 10 minutes. If it is the MAF that is the problem, before paying around $720 to the dealer to have them replace the MAF, there are 3 approaches you can take to lower the cost: 1. Reset the codes. Since a single “not correct” signal will set the CEL, see if it ever comes back. Not recommended if the car isn’t running right 2. Remove the existing MAF and clean it. You’ll need a specific Tamper Proof Torx bit to remove it and some electronics cleaner to clean it. Typically, you can clean the MAF once or twice and drive for 20k miles in between cleanings before having to replace the MAF. Total cost ~$20-35 for the initial cleaning, $0 for subsequent cleanings. 3. If that doesn’t work, since you already are so proficient at removing and reinstalling the MAF because you did the cleaning, buy a MAF mail order for much less than the dealer will charge and install it yourself. Total cost ~$265-295. Any time you replace the MAF, you need to disconnect the battery for 5-10 minutes (have your radio code handy) so that the DME unlearns the settings from the old MAF and is forced, on startup, to learn the settings from the new one. Number two is relatively easy, cheap, can be redone again for no cost (because you now have the cleaner and tool) the next time it is needed, and does no harm. And you can do number three if number one fails to cure the problem. What Can a Dealer Do That You Aren’t Liable to Be Able To Do? The dealer has access to the repair manual. The manual has tests the dealer can conduct using a multi-meter to see if the wiring coming to and from the MAF is bad or if there is some other computer problem.
oh it's very interesting,thanks manel for this article,my english isn't perfect but y can understand all. Best Regards
