Help with engine codes 94 Grand Am

94 Grand Am 3.1 V6 auto
Just to recap the problem. Engine light comes on when coasting at highway speeds, and the speed drops from 120-110 km/h. Doesn't happen at all under
90. ALso resets after turning car off. Anyways the garage has been unable to fix the problem. They gave me the following codes.
33 34 51
They said something about 51 being a prom error, and they said they replaced the prom, so it can't be that. Also something about the other 2 being voltage sensor readings or something. Can someone offer any help in identifying these codes for me, and also any possible solutions.
Regards, KDawg
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Also wanted to add that they replaced map sensors but to no avail.
Regards, KDawg

to
replaced
something.
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(when in doubt...) Have you cleaned the EGR passages?
Might be an intermittant vacuum leak. (33 and 34 are MAP high Voltage and MAP low voltage)
GW
KDawg wrote:

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When you let off the gas and your cruising with a warmed up engine the egr valve should open at times. Maybe the egr has a role in this?

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They said they checked for vacumm leaks and found no problems. They also replaced the map sensor, and did O2 sensor as well. Replaced the prom, but I'm not sure if that is the same thing as replacing the ECM.
I unhooked the battery last night for about 45 minutes. Hooked it back up, and took it out for a test. I though the problem had vanished cause it did not happen for over an hour of driving and testing under the conditions that would usually flag the sensors. Then it finally came on later last night.
Regards, KDawg

highway
under
unable to

replaced
something.
any
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The PROM is just a chip or module that plugs into the ECM.
--
Robert Hancock Saskatoon, SK, Canada
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Code 51 basically means some kind of internal failure within the ECM. If it's not the PROM, it's likely the ECM itself.
Code 33 is "MAP sensor signal voltage was too high (low vacuum) for 4 seconds when throttle angle was between 3% and 30% and held steady." Code 34 is "MAP sensor signal voltage was too low (high vacuum) for 2.5 seconds when engine speed was above 500 RPM."
Often when you have contradictory trouble codes set like those two, that tends to point towards a bad ECM as well..
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