P0118 code - somebody please tell me what the deal is

Zackj46

New Member
Jun 13, 2022
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Atwood il
So I bought another mustang, it’s a 2004 gt. I bring it home and I notice the temp gauge and the ac wasn’t working, so I look at the ect sensor and see it was ripped apart. After replacing it, everything works again. It still idles very rough and if I rev it to 6k it starts sounding like it’s misfiring, if I run it full throttle to redline it falls on its face as soon as it gets back down to idle.
It originally threw the codes p0118 and p0175 but after running full throttle once it throws p0172 as well. It also has a ticking sound coming from what sounds like a cylinder on the drivers side, the previous owner said it was blow by from an injector and it began after he replaced them.
Another thing I’ve noticed is the plug ect sensor, when the car is turned on it reads .065 volts from the power wire and .005 from the ground signal wire. I haven’t found anything online that talks about this problem but I’m assuming it’s a bad ground? If anybody can help me out it would be appreciated, I’m about to start tearing it apart but I’d like some guidance on where specifically I should start. Thanks guys
 

wmburns

SN Certified Technician
Aug 14, 2009
5,894
521
204
Houston Texas
.005 from the ground signal wire. I haven’t found anything online that talks about this problem but I’m assuming it’s a bad ground?
If we assume the above reading is from the ECT signal wire and battery negative the 0.005 volts is a very small difference. IMO this isn't likely to be your problem. However IF you are looking for a better way to know for certain, IMO the smart answer is to use an ODB2 scanner that is capable of monitoring PCM operational data. Simply "ask" the PCM what the ECT value is. After a cold soak the ECT value should match the local air temperature.

Here is some information on an affordable advanced ODB2 scanner that is within anybody's budget. No excuses.

ForScan ODB2 scanner w ELM327 USB​


If you still think that your problem is a bad ground, the try using a set of jumper cables from say the alternator case directly to battery negative. Does this change the ground reading at all? Or even better perform the voltage drop measurements as outlined in the resource link below.

Howto perform charging system voltage drop test​


Finally look for a wiring fault to/from the ECT.

Diagnostic Trouble Code (DTC) Descriptions
P0118 - Engine Coolant Temperature (ECT) Circuit High Input
  • Open circuit in harness
  • Sensor signal short to power
  • Damaged PCM
  • Improper harness connection
  • Damaged sensor
 
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