A very challenging diagnosis(P16A7 E46 2004)

I feel that I have to do a blog on this one. I almost met my match as to a diagnosis and repair. Currently the 2004 BMW 325i is not fixed but I know exactly what is wrong and the verification and repair is slated for Monday.

The vehicle in question came in to us on Wednesday. It had been to numerous shops and had a very perplexing issue. While driving the Check engine light, the EML light, The DSC, ABS and Brake warning light would come on, the vehicle lost all power and the accelerator pedal refused to respond. The customer had already replaced the MAF and the IAC (Idle air control motor). We read codes for DME-DSC CAN interface, and the infamous P16A7 MAF signal implausible code. I ran through the Test plan in the GT-1 and the MAF sensor read properly. I ran through another test plan that tests all major sensors and all were correct and within proper operating range with the exception of the LOAD (mg/stroke) and there was a negative fuel trim on one bank and a positive on the other. Bank 1 was at 3.48% and bank 2 was -1.67%. At idle it’s supposed to be between 40-80 mg/stroke(the load reading) and the mixture adaptations should be 0-8%. We were reading 0 on the load measurement. Furthermore, on the test drive the vehicle was severely underpowered, and would respond very anemically to throttle inputs. It was just slow and power was in the range of a Hyundai 4 cylinder.

The real issue was when we were climbing up a hill. The lights would come on (see above) and then it ran as if the engine was in failsafe mode. It’s a manual transmission so manipulation of the gear ratios allowed me to still nurse her back to the shop, and if left running the ONLY code that came up was the 16A7 MAF signal implausible-currently present. Once the car was shut off, it returned to operational but the car still severely lacked power. Also, none of the lights came back on. It was like it never ran improperly.

So, we checked all wires going to the MAF from the DME. Voltage was correct and all other wires had continuity. I mean 0 Ohms, not even .1 which is typical and expected due to the resistance in the wires.

So we loaded what is now the proverbial “shotgun” and fired a DME into the car. (This decision was not made without some serious discussion with the customer as to the likelihood of it’s possible reluctance to work and fix the car but it’s all we had to go on based on the faults and wiring checks as well as the BMW test plan from the GT-1) We received the DME the next day, coded and programmed it and lo and behold, it didn’t work.

We started the head scratching once again and I performed as best a research I could on the internet to find similar cases and I found a few cases similar to this case but no solutions. It was up to me, Jeb and Tom. We had no help. It was up to us to figure this one out and get the word out to the masses. So here is what we figured:

The DME is a computer. It believes inputs and appropriately directs certain actuators to do what they do and make the engine perform as to the inputs from the driver. The 2004 uses AFM sensors, not typical O2 sensors. They actually measure the A/F(air/fuel) ratio coming out of the cylinder. This reading is sent to the DME and appropriate changes are made to the open time of the fuel injectors(changes fuel mixture). That’s common knowledge. Well, common to us mechanic types out here. But we take something for granted. We make assumptions. And that Fuel Trim difference has been bugging me. Like a thief about to steal my soul, lurking around the corner. Then it hit me like a train. The Catalytic Converters!  The exhaust did sound strange when driving, but it still drove and always restarted the instance it cut off. I bet they’re plugged.

So today, Jeb and I took out the AF sensors (Pre-CAT O2′s) and removed the hood insulation (didn’t want to melt anything) and took it out for another quick test drive around the lot. WHAT A DIFFERENCE! She was ALIVE! And with the O2′s out it allows the engine to properly exhale and make room for that clean air and fuel to mix stochiomectrically and VROOMMMMM! We were pretty sure it was the Catylitic converters but to be sure we need to remove the post CAT exhaust system (AKA the downpipe)and make sure someone didn’t shove a banana up the customer’s tailpipe. We’ll have our answer Monday but let me take the time to explain why we got stuck and how you can tackle this problem if it ever rears its ugly head at your shop.

The Code P16A7_HFM_SIGNAL_IMPLAUSIBLE code is set because the HFM has Sensor PRIORITY. The DME is programmed to believe that the HFM will never lie to it and it uses the AFM sensors to back it up. When the AFM sensors say “Hey man we’re running lean” and the HFM says “I have this much air coming in” the DME knows how much fuel it’s adding to the mixture and the bore and stroke and it applies the calculation and when things don’t measure up the assumption is that you have a vacuum leak. I’m guessing (and it is a rather educated guess) that the engineers that designed the programming for the DME left out the chance that the engine has been running rich and let this situation go on for so long that it allows the Catalytic converters to cinder.

A Catalytic Converter has three precious metals; platinum, rhodium and palladium. These metals have specific properties that take the unburned hydrocarbons (gasoline) and oxygenate them, changing them from poisonous gasses into stable safe gasses. They also change other combustion byproducts but I’ll get to those in another blog or you can look up Wikipedia online here(opens a new window). They do their job sort of in the same way that your stomach and digestive track “burn’ the food you eat. Calories are a measurement of heat. I’m sure you’re familiar with the term Calorie. Well those precious metals hold on to oxygen molecules and release them when needed, oxidizing the fuel and thus “burning” excess fuel. In the case where the cat has to burn too much fuel it gets so hot it melts these precious metals and since the cat is visually identical to a bees honeycomb, the holes close up clogging your exhaust system and prevent your engine from exhaling. If you could inhale all day but couldn’t exhale you’d be dead pretty quickly. So, the engine builds excessive backpressure and no more air can make it in or in this case, only enough air comes in and goes out to allow idle power so as to confuse us mechanics into believing, much like the DME that the HFM can’t lie. In this case, the car had sufficient flow thorough the Cats to operate at idle. Once you crack open that throttle the engine can’t exhale enough to keep up with the increase in incoming air, it gets stopped up, especially in increased load applications, the DME can’t believe it’s program, and waves the white flag in the form of the Christmas tree of lights in the cluster and the engine enters Failsafe mode.

I Hope if you come across this problem this article helps you. I believe it is the only explanation of its kind on the internet and I’m so excited that I can close this case as solved. Until the next big problem, this is Josh, signing off. Happy St. Patrick’s Day!


We replaced the Catylitic Converters today and man! that sure wasn’t the same car that came in here! It had all the power back and then some! Well it felt like it in comparison to when it came in. We were happy that this case was closed and we look forward to the next challenge.


March 17, 2012Permalink 14 Comments
14 Responses to A very challenging diagnosis(P16A7 E46 2004)
  1. chris says:

    im so happy i found this i been haveing the same problem


  2. Stephanie says:

    Thank you so much Josh. I have been fumbling through the internet for days trying to figure out what the problem was. My mom has a 2004 BMW 325i with 7 codes currently. Through lots of blood, sweat, and screaming, I think I can finally give my mom a working car again. Thank you again!


  3. joshua says:

    It’s my pleasure to help anyone deal with the difficult issues diagnosing these complicated machines. I’m glad to hear that your moms car is back on the road and hopefully she won’t be suck with any more issues. Drive safely and enjoy your ultimate driving machine!


  4. matt mckinney says:

    This was a lifesaver!!!
    working on a customer’s car that had three cylinder coils misfiring and air intake boot with a massive hole in it for who knows how long and other vacuum lines leaking so I changed the plugs, coils, intake boot and repaired vacuum lines and the engine ran smooth on the test drive then started to lack power just as you described. No doubt to the excessive amounts of fuel into the cat it must have cooked it! Thanks again this spared me the frustration of tracing this down.

  5. Harold Cohen says:

    Hi, i have the code p16a7 in my 04-330ci. we removed catalytic converter we cleaned put it back and nothing happen, car is worst than before.
    exhaust or air intake make more noise that other cars like mine.
    what can you suggest me??

  6. joshua says:

    You can not clean a catylitic converter. It must be replaced. Remove the pre-cat O2 sensors and test drive. If you notice a difference then it is obvious the Cat needs to be replaced.

  7. Sly says:

    Hi, so i am having the same issue. I was just wondering, once i take out the sensors, and it drives fine, does that mean it is the sensors or the Cats are bad. Here is the reason i am asking, i have these codes, P16a7, P0430, and P0420. The last two state that the catalyst system efficiency is below threshold, they are bank 1 and bank 2. Now i am wondering with these codes,if you could tell me which one it is. If it is the cats. Can i just delete the cats and go straight pipe. Also would i need to change the 02 Sensors?

  8. Mike says:

    Is this the same for a 5 series I have a 525 I 2004 and the same code came up and lost of power I cleared the code and it hasn’t happen again

  9. joshua says:

    Yes it’s the same. If it cleared and hasn’t retuened I wouldn’t worry about it unless it reoccurs.

  10. joshua says:

    The converters are integrated into the exhaust manifold. If you were to go and remove the manifolds, cut them open and remove the cindered catylist material then in essance you would have “straight pipes” as you suggested. The O2 sensors appear to operate correctly if they throw this code since the A/F ratio coming from the MAF isn’t congruent with what is being reported from the O2 sensors.(Before and after gasses being combusted)The exhasut gasses are being trapped behind the catylist and since the programmers at BMWAG didn’t consider a clogged cat in their OBD programming, this produces a code that indicates a problem with the sensors. But the sensors are doing exactly what they’re supposed to do, it’s just a restriction in the exhaust that creates a situation where the programming wasn’t set to nor designed to identify.

  11. Greg says:

    I have an issue with my 2000 740i having long cranks. After I drive for a while and cut the car off…lets say to run into the store really quick. When I come out it will crank and crank but will not turn over. If I let it sit for a little while it will give a long crank but it does turn over. Took it to a trusted shop and they couldn’t figure it out. I took it to the dealer and they couldn’t figure it out. They say fuel pumps are fine. Cam sensor fine. Crank sensor is fine. Dealer tried to give me the run around. Charged me an arm just to tell me they don’t know. Oh…tried to offer me some new vehicles to look at by the way. You guys come highly recommend so I will bring my baby to you. Something tells me it’s something simple that others have overlooked. Dropping it off tonight.

  12. bob says:

    This is a great diagnosis. However, why did the catalytic converter plug in the first place? They should last a very long time with a properly running engine, so something else here is the root of the issue. Too lean/rich of fuel mixture is a problem somewhere else and needs to be found. Replacing converters gets expensive.

  13. Van says:

    Hello Joshua,
    A friend of mine with a 2005 BMW 525i that has just about the same problem; the car start up and runs find when cold but after about 5 to 10 minutes of driving the car would lose all power and Engine Fault / Severe Engine Malfunction warning lights are on; idle would fluctuate rapidly from 300 to 1,300 rmp. Turn the car off for a few seconds and turn back on would reset everything back to normal and allow you to drive for a mile or so down the road before the symptom reappear. When we disconnect the MAF the car would almost run normally so we replace the MAF with a new one but the problem still not gone away. The OBDII code reader shows P16A7 along with P0102, P0113, P0131, P0304, P0313, P0411… these ‘other’ codes are not always show up but the P16A7 is always appear when the engine is in fault mode.
    The following components were replaced: O2 (all), two Catalytic converters (@headers), Secondary Air pump and MAF sensor. I check on RealOEM and it shows that this particular model has four Catalytic converters, two are at headers and two are a bit further down the exhaust pipes but when check I only see the two Catalytic converters at the headers.
    At this point we’re stuck and hope that you could provide some assistant and let us know if we’re overlook something.

  14. joshua says:

    Sorry I didn’t see this until now. Wow! That is strange.
    Like I said in my article the 16a7 code is a correlation code between the pre cat o2 sensors and the MAF. Is it possible you still have blockage in the exhaust? I have seen Catalyst material break off and end up further down in the exhaust tube. Furthermore, you have a misfire code on cylinder 4. Do you have any aftermarket equipment like a K&N air filter or one that is dirty? If no then check the ground circuit on the MAF and the DME. check alternator ripple pattern with a scope and check batter condition. There is a lot of things I would check to figure your problem out. It is very strange indeed. If you’re close bring it in. I’d love to ry to figure this one out. Sounds like fun.

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