1995 Mustang GT- No PCM Communication, Runs Rich, Lacks power

gtrob said:

Is there any way I can check ECM for internal fault?

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Here’s how you can check for codes if your scanner won’t communicate:

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How to Pull Codes on a 1995 Mustang GT Without a Scanner​

1. Locate the test connectors
   • Open the hood. On the driver’s side near the firewall (by the brake booster), you’ll see a small gray plastic connector (that’s the Self-Test Input, STI) and a black rectangular connector (that’s the Self-Test Output, STO).
   • The gray STI connector usually has a single wire with a white/red stripe coming off the main harness.
2. Get a jumper wire or paperclip
   • You’ll use this to connect the STI to the signal return (SIG RTN) pin on the larger black connector (it’s typically the single black wire with a white stripe, sometimes on a short pigtail next to the main connector).
   • Important: Do not accidentally jumper to the single dark brown wire with orange stripe nearby — that’s 12V for the underhood light and will fry the computer.
3. Turn the key to RUN (don’t start the car)
   • The Check Engine Light (CEL) on the dash will begin to flash. This is called the KOEO (Key On Engine Off) test.
   • Watch the flashes closely. They come in groups separated by a pause. For example: flash-flash (pause) flash-flash-flash = code 23.
4. Read the codes
   • Code 11 (flash–pause–flash) means the computer itself passed its self-check.
   • Other codes will follow in two-digit or three-digit format depending on your ECU (’95s are three-digit).
   • The codes will repeat twice. Write them all down.
5. Run the KOER (Key On Engine Running) test
   • Turn the car off, keep the jumper connected.
   • Start the engine, let it idle.
   • The CEL will flash once, then a pause, then once again (that’s code 111 = system OK).
   • Any other codes will then flash. Record them.

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If No Communication or No Codes​

• If you don’t even get a code 111 with the key on, the ground circuit may be open. The white/lt-blue wire at the STI is tied into the computer’s signal return, and if that ground is bad you’ll get no codes and the car may run poorly (weird idle, fueling issues).
• Check the small black/white ground wire coming off the battery negative cable — it bolts to the body near the starter solenoid. Clean and reattach it if it looks corroded.
• Also inspect the 10-pin connectors (“salt & pepper shakers”) at the back of the intake for corrosion or broken pins. A bad connection here can kill communication.
• If the wiring checks out and you still get no response, the PCM itself may have failed (common on SN95s when the internal capacitors leak).

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