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Took the car to a mech the other day (97 cougar 4.6) b/c ses light came on. He thinks that the pcm has bit the dust. Apparently all 4 o2 sensors "not grounded?" , he thinks that because the negative for the o2's are grounded through the pcm, is this true. Has this happened to anyone? car runs fine.

Thanks in advance for any reply
 

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Was a code scanning performed? There's a couple of different diagnostics routines one can follow, but knowing if there's a code or not, or what code was scanned, will assist in following the correct routine....
 

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I have a similar problem. I get codes for all of the O2 sensors, but I tried replacing them all and checking the harness. The last step is to replace the PCM, but the car runs perfectly.
 

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I was reading through the routines in the 1997 Service Manual, and came across this:

NB1 MALFUNCTION INDICATOR LAMP (MIL) ALWAYS ON: CHECK MIL CIRCUIT FOR SHORTS TO GROUND

Note: If vehicle will not start, go to Pinpoint Test StepA1 .

If any Key On Engine Off or Continuous Memory Diagnostic Trouble Codes are present, service before proceeding.

Key off.

Disconnect PCM. Inspect for damaged or pushed out pins, corrosion and loose wires. Service as necessary.

Install breakout box, leave PCM disconnected.

Measure resistance between Test Pin 2 (MIL) and Test Pin 51 or 103 (PWR GND) at the breakout box.

Is resistance less than 5.0 ohms?

Yes
SERVICE short circuit between Test Pin 2 and Malfunction Indicator Lamp (MIL). REMOVE breakout box. RECONNECT PCM. RERUN Quick Test.

No
REPLACE PCM. REMOVE breakout box. RERUN Quick Test.

Also, on the preliminary checks for the MIL light always on symptom, it states to verify the Fuel Filler Cap is properly tightened....
 
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