Here are comments on diagnosing and solving Audi I5 20V turbo engine misses and misfires FIRST - Look through this and figure out what it is doing. #1.    Does the engine run?      - yes, go to item #2 below. - no, Turn on the ignition key, don't crank the engine. Does the "Check Engine" light turn on? It should turn on before you crank the engine. - no. The fuse above the ECU (under the floorboard) is probably blown. You have to lift the passengers floorboard out to get at this fuse. It is right by the ECU connector. Another symptom of a blown ECU fuse is inability to read engine codes with a VAG-COM or similar device. - yes, ????? may not be ignition problem. More to be written on CPS, etc. Was the fuse blown? -yes. If it blows again as soon as you try to start the engine you probably have a shorted coil or a shorted wire going to the coil. Examine the wires that enter under the spark plug cover at the rear of the engine, this is a common point to have shorted wires. If the wires are not pinched by the rear of the sparkplug cover, try disconnecting the coils one at a time. When you disconnect the wire from the coil, insulate it so it doesn't short. After you disconnect one coil, replace the ECU fuse and turn on the ignition and try to start the engine. If the fuse did blow again, reconnect that coil and disconnect the next one. When you find that the ECU fuse does not blow when a coil is disconnected, that coil is probably shorted. If the engine runs, although with a miss, with the coil disconnected and the ECU fuse does not blow - you have probably found your problem. -no. ????? more to be written. #2.    Does the engine miss all the time, or just under boost? - all the time. You probably have a failed Power Output Stage (POS). Determine which cylinder is not firing properly. Remove the cover over the fuel rail. Let the engine idle. Unplug each injector, one at a time, until no difference in idle character is made with the injector either plugged in or unplugged. When you unplug the injector for the wrong cylinder it is obvious. You will be converting the idle of a five cylinder engine hitting on only 4 cylinders to a five cylinder engine hitting on only three cylinders. This is a quite noticeable difference. - just under boost. You probably have a weak coil, bad sparkplug, or injector problem. #3.    Recovering from a failed Power Output Stage. You can confirm that the dead cylinder is caused by a bad POS channel by swapping the two POS modules (assuming it is acting up*). If the problem moves to a different cylinder (or goes away in the case of cylinder 3), it is POS for sure. (i.e. problem would move between 1 and 4, 2 and 5, or 3 and fixed when POS modules are switched.) *If it has fixed itself again, don't worry, the problem will be back again, and will settle in to stay in a matter of a couple days! Parts of this are copied from a FAQ that Bob Myers (robert@s-cars.org) wrote for the vanished UrS4/S6 web site. Corrections to Bob's writeup were done by Eric Renneisen (eric.renneisen@cigna.com) Bob wrote that this basic technique was suggested by Jeff Hemmerlin. The Power Output Stage for the urS4/S6 is a three channel device which feeds a signal to the individual coils for each spark plug. Two POS units are required. This provides 6 (2x3) channels, of which only 5 are used. When one channel goes bad it is possible to disconnect the wires from the faulty channel and move them to the unused channel. Of course, this can be done only once. The second time it happens you are forced to buy a new POS. Perhaps that should be spelled PO$. The two POS units are mounted on the firewall behind the engine and slightly toward the passenger side of the car (left hand drive) under the black plastic shield. They are oriented such that they are upside down with respect to each other. Remove the black plastic shield. The POS for cylinders #4 and #5 was toward the passengers side of the firewall on my '95 S6. The POS for cylinders #1, #2, and #3 was more toward the center of the firewall. You can unplug the smaller connectors that each have three pins. One will have three internal connections and the other will have two and an empty spot. That empty spot marks the unused "spare" channel. This should be on the POS toward the passenger's side of the firewall. That spare channel is for "cylinder #6". The other two connections of that plug are for #4 & #5. Depending on which cylinder is failing, you can figure out which POS has a failed channel. If the POS with the failing channel is the one driving cylinders #1, #2, and #3 then you will have to exchange the POS units. You have to have a POS with three good channels to drive three spark plugs. You want to use the POS with the failed channel to drive cylinders #4 and #5. If it was cylinder #3 that was failing, just swapping the POS units will fix the problem, since the failed channel will be in the spare position. If not, or if it was cylinder #4 or #5 failing then you need to rewire the connector plugs to use the two good channels for cylinders #4 and #5. Each POS has two connectors hooked to it. The three pin connector discussed above plugs into the output side of the POS. The other connector to that POS is a four pin connector that plugs into the input side of the POS. In the four pin input connector, there are three inputs from the ECU that correspond to the three outputs to the coils. The fourth pin is for a brown/white ground wire. The ground wire is in the input connector position #2 and should not be moved. Position #1 corresponds to output #3 on the 3-pin connector. Position #3 is for output #2 and position #4 is for output #1. ( for interest, input signals on pins #1, #3, & #4 are hot signals from the ECU. The POS controls the ground connections for each of the coils. So the POS outputs to the 3-pin output connector are either ground or open to fire the individual coils ). To rewire the connectors of the cylinder #4 and #5 POS. This is the wiring schematic into and out of the POS: In this example, assume cyl #5 was misfiring, so swap the wire from pin #2 to #3 on the 3-pin output connector and pin #3 to #1 on the 4-pin input connector. N127 Stock Changed Input POS Output 4-pin 3-pin #1 <--------> #3 Unused Coil #5 #2 <--gnd #3 <--------> #2 Coil #5 Unused (bad channel) #4 <--------> #1 Coil #4 Coil #4 After determining which of these black coil wires is to be moved pull the plug containing that wire and peel back the rubber boot covering the wire end of the plug. Using a sharp knife blade, gently pry the retaining clip holding the inner parts of the plug away from the retaining pin and slip the innards out of the plug. It will come out easily as a smaller (probably hot pink color) plug piece. You will see small snap levers which hold the individual connectors inside the main unit. Use the "special tool" (consisting of a couple of ~1mm by 10 mm flat steel pins which slip into the connector piece just like it was the mating half of the PSO). This frees the pin up so that it will slide out by pulling the wire. The tool is available at fairly reasonable price from Snap-On but could probably be Jerry-rigged from a thin piece of metal, or a very small jewlers screwdriver. Top of Audi page Mail to Charlie 5020 visits since This information and pictures copyright © Charles Smith, 2006.