Question about 1999 Volkswagen Passat

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Check Engine Light On idling rough once car is warm

Scan tool returns the following codes 18010; 16684; 16686; 16685; and 16502. I changed all four ignition coils, spark plugs, and power transistor. What is the problem and how can I fix this myself. Thank you in advance for any assistance rendered.

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  • d0sitmatr
    d0sitmatr May 11, 2010

    what year is the car and how many miles ?
    idling rough coupled with the misfires could mean a timing problem, if your miles are getting up there and you havent had the timing belt replaced, it might be a good time to consider it. if it breaks while driving, it could cost you up to 4 times as much as just replacing it ahead of time.
    Im going on what I know of Audi/VWs in the past, if they have changed something in recent yrs that deals with the timing belt not ripping all the valves off when it snaps, then it may only cost you a little more, but regardless, if it breaks while driving, it will cost you more.



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Your codes are as follows:

18010: "ECT Circuit High Input"
16684: "Random Misifre"
16685: "Cyl 1 Misfire"
16686: "Cyl 2 Misfire"
18010: "Voltage Supply Terminal 30 Voltage Too Low"

Your codes 16684, 16685 and 16686 are symptoms, not the problems. Obviously your engine is running roughly. So, once you get the problem fixed, these codes should go away on their own.

The 18010 code is normally displayed after the vehicle's computer has been re-set or replaced. If you've recently attempted to reset the computer in an effort to fix your issues, don't worry about this code. It too will go away when you get the main problem fixed. If you haven't reset the computer (or disconnected the battery for any other reason), then you'll want to check your battery cables and the power source cables running to your computer to make sure they're connected securely and are free of corosion.

Having said all this, we get to the root of your problem (code 16502). This code is triggered when your Engine Coolant Temp Sensor goes bad. The sensor typically sells for $60.

To remove and replace it:

  1. Allow the engine to cool below 100°F (37°C) before working on the cooling system.
  2. Note the anti-theft code and radio presets for the radio.
  3. Disconnect the negative battery cable.
  4. Locate the Engine Coolant Temperature (ECT) gauge sending unit.
  5. Drain the engine coolant until it is just below the level of the sensor into a sealable container.
  6. Detach the gauge sending unit electrical connection.
  7. If the sensor is held into the coolant passage with a retaining clip:
    1. Remove the sensor retaining clip, and pull the sensor straight out of the housing.
  8. If the sensor is threaded into the coolant passage:
    1. Loosen the sensor using a crowfoot wrench, a suitable boxed end wrench or a thin-walled 3/8 inch drive 12 point deep well socket and breaker bar.
To install:
  1. If the sensor is held into the coolant passage with a retaining clip:
    1. Install a new O-ring on the sensor.
    2. Using a small amount of fresh coolant or an anti-seizing compound, lightly lubricate the O-ring, and push the sensor in the housing.
    3. While pushing inward on the sensor, install the retaining clip.
  2. If the sensor is threaded into the coolant passage:
    1. Install the sensor with a new crush washer. Tighten the sensor to 12.5-14 ft. lbs. (17-19 Nm).
  3. Plug in the harness connector to the sensor.
  4. Refill the cooling system with the proper type and mixture of coolant and bleed as necessary. For additional information, please refer to the following topic(s): General Information and Maintenance, Fluids and Lubricants.
  5. Reconnect the negative battery cable and re-code the radio as necessary.
  6. Start the engine, allow it to reach operating temperature and check for leaks.

Hope this helps and good luck!

Posted on Jan 12, 2009


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