MAP sensor - weird behavior


New member
Hello everyone,

My MAP sensor has triggered an CEL the second time, so I figured I'd ask about it..
First time it was near sea level, very hot outside, coasting down a hill. When I stopped the check engine was on, error code was P1270. FSM says it's a boost error. Torque pro showed 0.6 psi at idle and with the ignition on (engine off). Eventually I unplugged the sensor and plugged it back in and it was back to 0.0 psi at idle and with the ignition on.

This weekend it happened again, after climbing up a mountain but it showed 5.4 inches of vacuum. I tried unplugging and plugging the sensor back in and it did not help. After a while it dropped to 4.8 and the CEL cleared itself. Eventually with the ignition on and engine off it showed 2.8 inches of vacuum and it stayed there. Wiggling the wires didn't do anything.

With the sensor unplugged it goes up to 24.3 inches of vacuum, I can't remember what it showed in the summer with it unplugged.

Are MAP sensor failures common?

Oh yeah, this is were the CEL showed up. We got up, idled for a few minutes then when I restarted the engine, bam, check engine. I aborted the climb :(
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Engineer In Residence
It could also be the wiring. I believe the ECU supplies a reference voltage and measures the current flowing through the sensor to determine the pressure. So if the connection or the wiring is bad the result is a faulty reading. Pressure sensors are fairly simple devices but they do occasionally fail. Your intermittent operation makes me think it may be related to the connector or the wiring more than the sensor itself.

In similar situations I will disconnect the main harness from the ECU and measure the resistance of the sensor directly from the ECU plug. This can be compared against published values if they are available.

My first suggestion would be to clean the harness and plug with some type of electrical contact cleaner. Then I would inspect the harness for damage. If you continue to have issues the sensors are cheap enough I would suggest replacing it.


New member
Thank you, that is the plan, to disconnect at the ECU and measure. The first time it happened a disconnect-reconnect at the sensor fixed it. I figured I'd ask, maybe a sensor failure is common and I didn't notice discussions about it.


Well-known member
Just a few comments to make.
Back in 2004 Dodge issued a bulletin about defective ambient temp sensors as communication issues to the PCM affecting boost pressure performance .

In short incorrect or corrupted signals from this sensor seriously affects boost pressure and other engine controls. The reason is that although the PCM has the ambient air pressure sensor buried inside the PCM its computed effectiveness is determined by ambient air temperature which finally determines the actual density of the air being drawn into the engine.

MB is not the only manufacturer that does this, even Ford since the 7.3 Powerstroke days had the ambient air pressure in the PCM ('95/96 models) then put it out under the dash and continued to use an ambient air temp sensor high up near the radiator to compute the same as MB et al.

In passing I might also add that in the later 906 platforms the same criterion applies but it has much more drastic results such as negative actions of the EKAS and EGR valve etc.
For that reason alone this sensor should be checked at service intervals with a scanner to determine if it is performing accurately.

May I also suggest in passing that those who would contemplate buying a "Tune" (horrible word) to negate persistent EGR and EKAS problems might like to check this vulnerable sensor as the root cause of the issue, Cost of this sensor??? $27!


Erratic Member
Just a few comments to make.
Back in 2004 Dodge issued a bulletin about defective ambient temp sensors as communication issues to the PCM affecting boost pressure performance .
And did they mention that to me as i made 3 warranty visits (one per year) to replace the IAT sensor?
Nooooooooooo.... :yell:
(i even paid for the 3rd one, because i didn't want to wait while they fixed it)
((which finally let me investigate the faulty and new parts))



New member
Dennis, I know in my car I have two air temperature sensors: one is the ambient temperature, which is used (among other things I guess) to show the outside temperature, and the intake manifold temperature sensor. The defective ones are the ones on the intake manifold?

Updates on readings: 0.7 inches of vacuum idling, 1.6 inches with engine off and ignition on. This doesn't trigger a CEL.

This weekend I will hopefully visit my mechanic friend and try out some other intake temperature sensors and map sensors.

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