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Joseph Maduzia

1999 986 Boxster - P0112 Code Troubleshooting

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1999 Porsche Boxster 986 shows check engine light (CEL). Code read at autozone as P0112 Error Code (Intake Air Temperature Circuit Low Input).

According to <http://repairpal.com/obd-ii-code-p0112>, the error code causes could be

  • Defective Intake Air Temperature Sensor (IAT)
  • Dirty air filter
  • Defective Mass Air Flow Sensor (MAF)
  • Faulty or corroded Intake Air Temperature wiring or connection

MAF: Bosch 996 606 123 00​ (serial number?:0 260 217 007)

DME: Bosch 996.618.601.04 (serial number?:0 261 204 605). Lots of other numbers on DME. Can post image if required.

After doing some research, I did some basic digital multi-meter tests between the MAF, IAT, and DME (MAF/IAT are combined into one unit in the '99 Boxster).

NOTE: Engine Control Unit (ECU) seems synonymous or very related to DME or Digital Motor Electronics in the Porsche Boxster, Boxster S Service Manual: 1997-2004 by Bently Publishers {ISBN-10: 083761645X -- ISBN-13: 978-0837616452}

I detached the MAF cable from the MAF, and I uninstalled the Engine Control Unit (ECU) from the trunk wall and detached the ECU cable from the ECU. I then measured the resistance between the following terminals. These results were:

  • MAF 1 to DME 15: 0-0.5 Ohms
  • MAF 2 to DME 54: 0-0.5 Ohms
  • MAF 3 to DME 45: 0-0.5 Ohms
  • MAF 4 to DME 47: 0-0.5 Ohms
  • MAF 5 to DME 17: 0-0.5 Ohms

Based on the pinout of the MAF to DME, and according to Porsche Boxster, Boxster S Service Manual: 1997-2004 by Bently Publishers, I have a DME version 5.2.2. I'll have to update what the service manual descriptions of these terminals are tonight when I am near the book again.

Then, with the MAF re-installed and the DME re-installed and all cables reconnected, I measured the following values:

  • Engine @ idle, MAF 4 to MAF 3 is 5v.
  • Engine @ idle, MAF 1 to MAF 3 is 3volts
  • Engine @ idle, MAF 2 to MAF 3 is 13.6volts.
  • Engine @ idle, MAF 3 to car ground is shorted.
  • Engine @ idle, MAF 3 to MAF 5 is 1.38V.
  • Engine @ 3000 RPM, MAF 3 to MAF 5 is 2volts.

From these measurements, it appears as though the MAF is running properly. Could someone confirm?

Other tests/info:

  • I cleaned the MAF with MAF cleaner, which had no effect.
  • I drove the car with the MAF cable removed from the MAF and the car drove exactly the same as with the MAF cable connected.
  • I bought the car with the P0112 error code showing, so I don't know what the car drove like without it. That said, the car does not have difficulty idling and is definitely faster than my 2008 Corolla LE. I can't say if it is as responsive or as quick as it should be, however.
  • To try to find vacuum leaks, I sprayed carburetor cleaner on literally everything I could see in the engine with the top engine cover removed. the engine speed did not change from idle at any point during the carburetor cleaner test did.

With the engine off, the MAF removed, and the air temp around 60-70°F, I measured the MAF terminals below.

  • MAF 1 to MAF 3 is 1.7 kOhm.
  • Maf 2 to everything is either open loop or megaohms.

Then, with engine still off and the MAF removed, I breathed on the thermistor and read the following values:

  • MAF 1 to MAF 3 is 1.4kOhm
  • Maf 2 to everything is either open loop or megaohms.

Based on a DME 7.8 manual, the IAT is supposed to read 2.3-2.7kOhms at ~20°C. Which would put the resistance of my IAT (1.4-1.7kOhms) a little low. Does anyone have a DME 5.2.2 manual to confirm what the resistance of IAT (pin 1 to 3) should be? If this resistance is OK, then does anyone have ideas as to what else the problem could be? It seems like my current options are:

​I've read bad things about using after-market MAF's, so I will only be replacing w/ official MAF if that is the correct path to take.

Edited by Joseph Maduzia

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1999 Porsche Boxster 986 shows check engine light (CEL). Code read at autozone as P0112 Error Code (Intake Air Temperature Circuit Low Input).

According to <http://repairpal.com/obd-ii-code-p0112>, the error code causes could be

  • Defective Intake Air Temperature Sensor (IAT)
  • Dirty air filter
  • Defective Mass Air Flow Sensor (MAF)
  • Faulty or corroded Intake Air Temperature wiring or connection

MAF: Bosch 996 606 123 00​ (serial number?:0 260 217 007)

DME: Bosch 996.618.601.04 (serial number?:0 261 204 605). Lots of other numbers on DME. Can post image if required.

After doing some research, I did some basic digital multi-meter tests between the MAF, IAT, and DME (MAF/IAT are combined into one unit in the '99 Boxster).

NOTE: Engine Control Unit (ECU) seems synonymous or very related to DME or Digital Motor Electronics in the Porsche Boxster, Boxster S Service Manual: 1997-2004 by Bently Publishers {ISBN-10: 083761645X -- ISBN-13: 978-0837616452}

I detached the MAF cable from the MAF, and I uninstalled the Engine Control Unit (ECU) from the trunk wall and detached the ECU cable from the ECU. I then measured the resistance between the following terminals. These results were:

  • MAF 1 to DME 15: 0-0.5 Ohms
  • MAF 2 to DME 54: 0-0.5 Ohms
  • MAF 3 to DME 45: 0-0.5 Ohms
  • MAF 4 to DME 47: 0-0.5 Ohms
  • MAF 5 to DME 17: 0-0.5 Ohms

Based on the pinout of the MAF to DME, and according to Porsche Boxster, Boxster S Service Manual: 1997-2004 by Bently Publishers, I have a DME version 5.2.2. I'll have to update what the service manual descriptions of these terminals are tonight when I am near the book again.

With the engine off, the MAF removed, and the air temp around 60-70°F, I measured the MAF terminals below.

  • MAF 1 to MAF 3 is 1.7 kOhm.
  • Maf 2 to everything is either open loop or megaohms.

Then, with engine still off and the MAF removed, I breathed on the thermistor and read the following values:

  • MAF 1 to MAF 3 is 1.4kOhm
  • Maf 2 to everything is either open loop or megaohms.

With the measurements from MAF 1 to MAF 3 listed above, I believe the thermistor is at least operational. Whether it is operating properly or not is unknown to me.

Then, with the MAF re-installed and the DME re-installed and all cables reconnected, I measured the following values:

  • Engine @ idle, MAF 4 to MAF 3 is 5v.
  • Engine @ idle, MAF 1 to MAF 3 is 3volts
  • Engine @ idle, MAF 2 to MAF 3 is 13.6volts.
  • Engine @ idle, MAF 3 to car ground is shorted.
  • Engine @ idle, MAF 3 to MAF 5 is 1.38V.
  • Engine @ 3000 RPM, MAF 3 to MAF 5 is 2volts.

From these measurements, it appears as though the MAF is running properly. Could someone confirm?

Other tests/info:

  • I cleaned the MAF with MAF cleaner, which had no effect.
  • I drove the car with the MAF cable removed from the MAF and the car drove exactly the same as with the MAF cable connected.
  • I bought the car with the P0112 error code showing, so I don't know what the car drove like without it. That said, the car does not have difficulty idling and is definitely faster than my 2008 Corolla LE. I can't say if it is as responsive or as quick as it should be, however.
  • To try to find vacuum leaks, I sprayed carburetor cleaner on literally everything I could see in the engine with the top engine cover removed. the engine speed did not change from idle at any point during the carburetor cleaner test did.

Based on the tests done and the possible causes of the error code, I would say the DME software needs to be re-installed or a new DME needs to be purchased. From what I have researched, due to the security measures of the DME, I need to take the Boxster to the mechanic to have the DME re-flashed or replaced. But I wanted to see if any Porsche tech's out there had any feedback on this issue first. Perhaps there is another test I can do to hone in on the issue?

A lot of diagnostics effort. The P0112 code indicates that the intake air sensor in the MAF is faulty. Correct test is to pull the MAF and using a multimeter set to resistance (ohms) test across pins 1 and 3; you should read 2.3 to 2.7K ohms; you are only seeing 1.4K, so the MAF is out of range and needs to be replaced.

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A lot of diagnostics effort. The P0112 code indicates that the intake air sensor in the MAF is faulty. Correct test is to pull the MAF and using a multimeter set to resistance (ohms) test across pins 1 and 3; you should read 2.3 to 2.7K ohms; you are only seeing 1.4K, so the MAF is out of range and needs to be replaced.

Thank you for the quick response! I saw the 2.3 to 2.7 kOhm resistance criteria in a manual for 986 DME 7.8. Is the resistance criteria the same for DME 5.2.2? I imagine it is, but I just wanted to confirm.

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A lot of diagnostics effort. The P0112 code indicates that the intake air sensor in the MAF is faulty. Correct test is to pull the MAF and using a multimeter set to resistance (ohms) test across pins 1 and 3; you should read 2.3 to 2.7K ohms; you are only seeing 1.4K, so the MAF is out of range and needs to be replaced.

Thank you for the quick response! I saw the 2.3 to 2.7 kOhm resistance criteria in a manual for 986 DME 7.8. Is the resistance criteria the same for DME 5.2.2? I imagine it is, but I just wanted to confirm.

According to the Porsche OBD II diagnostic's manual, they are the same.

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Installed the new MAF. No change in error code. Refurbished ECU, no change in error code. ECU only intermittantly connecting, so replaced ECU with someone else's refurbished ECU (all through Specialized ECU Repair). Original P0102 code now gone, but now I get P1551. The original IAC read within specs according to mechanic, but replaced it anyway just to be sure. New IAC still shows P1551 error code. 

 

The only things I can think to test now are vacuum leaks (b/c they seem to be the secret cause of engine codes) and possibly trying to find a wiring harness to pin test all ECU pins while system is on with volt meter.

 

According to mechanic, the vehicle hovers around the speed when cruise control is set, and idle sometimes bounces up and down. Otherwise car seems to run exactly like it did before I replaced the ECU. I'm starting to believe that there is some ghost ECU or wiring problem. Any thoughts?

Edited by Joseph Maduzia

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Installed the new MAF. No change in error code. Refurbished ECU, no change in error code. ECU only intermittantly connecting, so replaced ECU with someone else's refurbished ECU (all through Specialized ECU Repair). Original P0102 code now gone, but now I get P1551. The original IAC read within specs according to mechanic, but replaced it anyway just to be sure. New IAC still shows P1551 error code. 

 

The only things I can think to test now are vacuum leaks (b/c they seem to be the secret cause of engine codes) and possibly trying to find a wiring harness to pin test all ECU pins while system is on with volt meter.

 

According to mechanic, the vehicle hovers around the speed when cruise control is set, and idle sometimes bounces up and down. Otherwise car seems to run exactly like it did before I replaced the ECU. I'm starting to believe that there is some ghost ECU or wiring problem. Any thoughts?

 

P1551 is a signal interruption in the IAC; most common causes are wiring breaks, bad IAC, or a problem with the DME.

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