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Old 05-20-2012, 18:49   #1 (permalink)
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1995 F-150 Check Engine Light On Code 172

I was recently given a 1995 F-150 4.9L six cylinder with Mazda 5 speed manual. 196,000 miles. Check engine light comes on after the engine has warmed up and I approach highway speed.

What's strange to me is that if I turn the truck off and then restart it the check engine light is not lit and does not come on until I accelerate to highway speed again. I'm under the impression that once the code is set the light will stay on until it is cleared??

Pulled codes from the computer with the engine off. Shows KOEO code 522 (not in park or neutral) and CM code 172 (HEGO sensor circuit indicates system lean right side).

Things I've done:

Checked the fuel pressure and it looks OK at 45psi. Checked vacuum lines etc. and see no leaks. Installed a new O2 sensor. Cleared codes. Light still comes on. Truck starts and seem to run fine.

Any suggestions are welcome!
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Old 05-20-2012, 20:04   #2 (permalink)
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Re: 1995 F-150 Check Engine Light On Code 172

Check the connection at the PCM (ECU).
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Old 05-22-2012, 16:05   #3 (permalink)
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Re: 1995 F-150 Check Engine Light On Code 172

Pulled the connector to the ECU today. Pins all look OK. Nice and clean with a light coating of dielectric grease.

Don't drive the truck every day. Will have to take it for a drive.
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Old 05-23-2012, 18:23   #4 (permalink)
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Re: 1995 F-150 Check Engine Light On Code 172

This is what happens when the code is not understood.
Lean means the computer table for that side has been shifted to it's lean side 'out of limits', setting the code.
Now the question is why?
Suppose there is an air leak in the intake tract at some point or excessive fuel from a leaking injector? This 'upsets' the expected air to fuel ratio.
Of course the system will take some time to detect this condition when the driving cycle varies.
The OX sensor detects this as the 'RESULT' of the problem.
The OX senaor is NOT the problem but what it detected is.
Quite a different situation than you are thinking
Good luck.
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Old 05-23-2012, 18:54   #5 (permalink)
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Re: 1995 F-150 Check Engine Light On Code 172

Yes, thanks. That's pretty much what I've been reading. Intake air leak, clogged injector, low fuel pressure, leaky vacuum line etc. Not an easy one so far.

Shouldn't the check engine light stay on after the code is set???

As I mentioned, the light isn't on after restarting the truck. Comes on again after warming up and driving for awhile.
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Old 05-29-2012, 19:10   #6 (permalink)
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Re: 1995 F-150 Check Engine Light On Code 172

The OX sensor is seeing a shifting oxygen condition and shifts the fuel tables in response.
As the fuel tables goes out of limits and back again or the problem comes and goes the CEL will come on as a result then off when the trouble clears.
No the CEL does not lock on if the issue is not present otherwise you could end up chasing an issue that doesn't exist.
You already have the code to work with so the CEL is no longer usefull unless you do a clear of 'old history' after the work, to see if or what code come back.
Good luck.
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Old 05-29-2012, 22:07   #7 (permalink)
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Re: 1995 F-150 Check Engine Light On Code 172

Thanks.

I'm seriously thinking about pulling the injectors out for a good cleaning. The truck sat for a year or so prior to my obtaining it.

Can't find anything else wrong with it.
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