Engine ACT Code 54 "Sensor circuit open"


New Member
Jun 4, 2008
Redwood City California
Hope this finds you all well.
What I have is a ’93 5.0 stroked to 347. It’s in a 1993 Volvo 940 wagon. Tuned a few years ago by the engine builder at his Sears Point shop.
It passed CA smog checks without too much sweat. The last few weeks I've been occupied doing some engine compartment clean-up. Removing most of the old production line connectors and cleaning up the vacuum hoses. It is still EEC-IV.
After everything was back together, before initial start up I clipped in my Innova EEC-IV Ford code reader.

KOEO: I get a 15 (EEC (PCM) Read Only Memory (ROM) test failed)
then a 54 (Intake Air charge temperature sensor circuit open)
The amazing thing is code 11 shows up after the second group finishes.

Check engine light is ON

The ECU is an X3Z.
I ran a new wire from the ACT sensor (also new) to pin 25. VREF is 5.05 vdc
Temp: 60F Sensor voltage is 4.76 vdc Resistance across sensor terminals is 42.6 K ohms. Both readouts appear to be in range.
Two new Motorcraft O2 sensors installed.
I drove the car yesterday. Seemed to run ok. CK Engine Light on the whole time…
Not sure to how to determine if the ACT sensor data is being seen by the ECU considering the last code seen is a 11
. IMG_5241.jpg
If anyone can think of anything else I might try…

Best regards all...
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SN Certified Technician
Mar 10, 2000
Dublin GA
Code 51 Engine Coolant Temperature (ECT) sensor signal is/was too high -

[color= blue]Revised 6-Apr-2017 to add diagrams and resistance check for ECT wiring.[/color]

Possible bad ECT sensor or wiring. Possible missing signal ground – black/wire broken or bad connection. With the power off, measure the resistance between the black/white wire and battery ground. You should see less than 1 ohm. Check the same black /white wire on the TPS and MAP sensor. More than 1 ohm there and the wire is probably broken in the harness between the engine and the computer. The 10 pin connectors pass the black/white wire back to the computer, and can cause problems.

The computer Engine Coolant Temperature sensor has absolutely nothing to do with the temperature gauge. They are different animals. The ECT sensor is normally located it the passenger side front of the engine in the water feed tubes for the heater. It has two wires that connect by a weathertight plastic connector.

The water temperature sender for the temp gauge is located in the driver's side lower intake manifold. It has a single wire that connects by a push on connector on the temp sender.

If you have replaced the ECT sensor and are still having ECT like problem symptoms, check the ECT wiring .



See the graphic for the 10 pin connector circuit layout.

Check the resistance of the green wire on the ECT connector to the green wire on pin 7 of the computer connector. You should see less that 1 Ω (ohm)

Use Pin 46 on the computer for ground for both ECT & ACT tests to get most accurate readings.

Pin 7 on the computer - ECT signal in. At 176 degrees F it should be .80 volts

Pin 25 on the computer - ACT signal in. At 50 degrees F it should be 3.5 volts. It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold, the voltage readings will be lower because of the heat transfer.

50 degrees F = 3.52 v
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.

50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms

Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/





New Member
Jun 4, 2008
Redwood City California
The ACT is located on the lower intake manifold. I may have stumbled across the reason for the 54 code. I removed the ACT sensor, checked the shared signal return to pin 46 resistance. 1.19 ohms. Not good... Clipped it off ACT cannon plug and jumped it to the ECT signal return. Ran Self-Test... no codes!


New Member
Jun 4, 2008
Redwood City California
Yes, it was tuned for the MAF. Not sure if the X3Z is the best choice... I became caretaker to the car when the gentleman who originally swapped in the engine maybe 20 years ago passed away in 2007. It was was set up for autocross and track driving. I would like to be more involved in the tuning aspects. The whole process kind of intimidates me. The Tweecer RT interface is in place- but I have yet to use it.

Last edited:


15 Year Member
Mar 31, 2005
The ACT is located on the lower intake manifold. I may have stumbled across the reason for the 54 code. I removed the ACT sensor, checked the shared signal return to pin 46 resistance. 1.19 ohms. Not good... Clipped it off ACT cannon plug and jumped it to the ECT signal return. Ran Self-Test... no codes!
Not sure I'm following... are you suspecting the ACT plug, or the ACT itself?

If the tune was done properly, then it doesn't matter what factory ecu you start with. Most factory differences were minimal anyway.


New Member
Jun 4, 2008
Redwood City California
Thanks for responding- I suspect my talents as a wire splicer...and an inexpensive replacement ACT sensor. Perhaps I did not pay close enough attention to my splices. What started all of this was I removed all of the original production wire harness connectors. Tested and mapped out the pin locations for future reference. But I failed at checking the quality of my work. Lesson learned.
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