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I was driving my '07 Forenza the other night and there was a loud popping noise at the same time as the check engine light came on. There was only one code: P0128 - Coolant Temperature Below Thermostat Regulating Temperature. It overheated to the point of smoking (I quickly shut it off) and neither of the fans are coming on.

I checked the fuses and relays in both fuse blocks, and they're all working fine. The only other damage I noticed is that my crankshaft position sensor cable has been worn almost through - I had my valve cover gasket replaced back in October and it looks like they just let the cable hang instead of clipping it back into position. The serpentine belt wore the insulation off and it must have shorted.

I'm wondering if this short is what could have triggered the code, and other cooling problems. I don't know if I can replace the CKP myself - I'm not afraid to get my hands dirty, but I don't have a lot of experience or many of the necessary tools to remove the compressor and power steering pump.

Is there anywhere else in this system I should look for damaged fuses and relays to replace before I take it to a shop? If I replace those and fix the CKP cable, is there a chance it will work again? Thanks in advance.
 

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The CKP (crankshaft position sensor) should have nothing to do with overheating.. If this cable was damaged to the point of shorting or grounding out your engine wouldn't even be running - or it would be very rough possibly intermittently. Certainly its not good that your CKP cable is worn so badly against the belts, and it should be replaced at their expense. However, that is not what is causing the overheating problem.

That code you pulled P0128 essentially means the ECU is never even seeing the engine get up to operating temperature let alone overheating. What is your temp gauge on your dash doing? Is it even working? My guess is that it just stays totally in the cold region or doesn't even move at all. This sounds like a problem with the coolant temperature sensor itself or the connection to the sensor or the wiring in between the sensor and the ECU. The sensor is resistance based, so any loose or corroded connections will significantly mess with the readings. I would suggest inspecting the sensor first and its connector. Its located on the driver's side of the head, below the ignition coils. Its a two wire plug.

Also if you got the OBD2 code read I'm assuming you have an OBD2 reader... you should be able to monitor your engine's parameters in real time (such as RPM, air temp, coolant temp, etc). You can try looking at the coolant temp and see what its reading in there... that will be the true value that the ECU is seeing and uses to determine when to turn on the fans. If you watch that value in real-time while moving/wiggling the connector and the reading starts to jump around all over, then you've got a bad connection.

Start with those things and let us know how it goes... Knowing what the ECU is seeing is the most direct way to start troubleshooting this.
 
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