Question about Ford F-150

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Hi, 4.2L. Check eng light On. I have a code P0125, and temp guage isn't moving off cold. I understand the code as the coolant isn't coming up to proper temp in the specified time. The coolant was at proper level. I just replaced the thermostat with a new 195deg, let the truck idle and the guage rose into the operating range. I erased the P0125 code, check eng light went off. However, the next morning the guage didn't move off cold again. BUT after 10 miles (ambient temp 80deg)I pulled over for a minute and the temp gauge started raising. Once back on the road, within 2 miles I was back to cold. At the next light, the guage rose again, but dropped once I got going. The check engine light came back on after approx 30 miles. Any suggestions? I am also confused as to where the coolant sensor is. The one I got from Napa and see on the web, a 2 wire, screw in, is nowhere to be found on my engine. ???

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  • Duane Jul 15, 2010

    Thanks Duane for your quick replies and advice.



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Removal & Installation for 2000 F-150, You didn't say which year you had, so I guessed 2000 as an example:
4.2L Engine 1997-2003 To Remove:

  1. Before servicing the vehicle refer to the precautions at the beginning of this section.
  2. Remove or disconnect the following:
    • The negative battery cable NOTE: When a battery is disconnected and reconnected, some abnormal drive symptoms may occur while the vehicle relearns its adaptive strategy. It may be necessary to drive the vehicle 10 mi (16 km) or more.
    • Some of the coolant NOTE: Clean coolant may be collected and reused.
      ECT sensor Hi, 4.2L. Check eng light - f150_42_temp-sen.gif

    • The electrical connector
    • The ECT sensor
To Install:
  1. Install or connect the following:
    • The ECT sensor
    • The electrical connector
      1. Torque to 89 inch-lbs (10 Nm)
    • The coolant

Posted on Jul 13, 2010

  • 2 more comments 
  • Duane Jul 13, 2010

    Hi Duane,

    Thanks for the quick reply and guess on the year, it is actually a 99. Can you help me locate the ECT sensor? The attached picture still didn't help me. It is probably in front of my eyes, but I can't find it. Is this the only type temp sensor Ford used on the 99 4.2?

    Is removing the negative battery cable a safety precaution or does it help with resetting the computer?



  • Duane Wong
    Duane Wong Jul 13, 2010

    In general I would say it's a safety precaution to protect the computer from any surges or shorts as you poke around the engine compartment.

    Look on the cylinder head or intake manifold. I'm sorry if the pictures did not show up on, as I think they try to limit pictures for bandwidth issues.

    Good luck to you in this endeavour.

  • Duane Jul 13, 2010

    Thanks Duane.

    I will look again and let ya know.


  • Duane Jul 15, 2010

    Hi Duane,

    Today I found the upper 1/2 of the sensor dangling, attached to the harness, in back of the engine. It is obvious it is broken, and the threaded 1/2 is still in the engine. The internal resister and wires are with the upper end of the sensor hooked to the harness. It appears to me that when the truck was stopped the wire would ground sending my guage's needle upwards. Once the truck was rolling the sensor would move away from the engine and the ground would be lost resulting in the needle falling. I have a head-banger of a task to locate the broken off piece of the sensor screwed somewhere in the back of the engine.

    Thanks for your help.




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