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.