Code 23 MAT
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Code 23 MAT
My check engine light comes on after I run the car, but if I crank it again it might not be on. It' s a code 23, MAT and I not sure were to locate the Manifold air temp sensor to diagnose the problem. Can anyone tell me where it is? I have the FSM but can find anything on it except the troubleshooting diagram. Is a SCAN tool needed to check it or is there another way? 
The FSM says the Code 23 is the MAT, but the Haynes manual says its the IAT. Are they the same thing? I know where the IAT is.
The FSM says the Code 23 is the MAT, but the Haynes manual says its the IAT. Are they the same thing? I know where the IAT is.
Last edited by RRT vette; Mar 22, 2006 at 08:19 PM.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Originally Posted by AGENT 86
It's threaded into the bottom of plenum. 
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Originally Posted by CFI-EFI
IAT = Intake Air Temperature.
MAT = Manifold Air Temperature.
BTH = Burn The Haynes
RACE ON!!!
MAT = Manifold Air Temperature.
BTH = Burn The Haynes
RACE ON!!!
Yeah the Haynes doesn't give as much info, but finding things is a little easier. Guest not for this. 
Instructor
Joined: Oct 2005
Posts: 108
Likes: 0
From: Acton, MA 1990 ZF6 Coupe, Z51
They are easy to check with a volt-ohm meter....Trace the two wires coming out of the sensor from under the plenum, they should terminate in a harness plug...On my 90 it's buried behind the distributor next to the firewall....Unplug and connect your VOM into the sensor half of the plug and set to ohms....
Basically it's a resistor which changes inversely to the manifold temperature..(as temp. increases resistance goes down and vice-versa)..Your trouble code is for out of range low, so your resistance is too high, anything above 15K and the sensor is junk...Start the car, as it warms up, the resistance should drop....If it doesn't get below 500 ohms, again, junk...
If the sensor tests ok, could be the harness wiring, check it ohms out to the ECM....
Basically it's a resistor which changes inversely to the manifold temperature..(as temp. increases resistance goes down and vice-versa)..Your trouble code is for out of range low, so your resistance is too high, anything above 15K and the sensor is junk...Start the car, as it warms up, the resistance should drop....If it doesn't get below 500 ohms, again, junk...
If the sensor tests ok, could be the harness wiring, check it ohms out to the ECM....
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Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
I found the MAT sensor. It has a tan and black wire . The wires are in a bent shape and the tan wire has the insulation cracking, exposing the wire, but not touching anything. Could this be part of my problem?
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Certainly could be - the easier way to check it out is to buy another one - about 9 Bucks. You don't have to install it - just plug it in. If the Code is still present, it's in the wiring or the ECM. If it goes away, replace it. Unfortunately, the easiest way to do that is to remove the Plenum.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Originally Posted by SunCr
Certainly could be - the easier way to check it out is to buy another one - about 9 Bucks. You don't have to install it - just plug it in. If the Code is still present, it's in the wiring or the ECM. If it goes away, replace it. Unfortunately, the easiest way to do that is to remove the Plenum.
That sucks I just install new gaskets on the Plenum.
Burning Brakes
Joined: Nov 2005
Posts: 848
Likes: 1
From: 1992 LT1 6 speed Cayman Islands
Originally Posted by JeffM1243
They are easy to check with a volt-ohm meter....Trace the two wires coming out of the sensor from under the plenum, they should terminate in a harness plug...On my 90 it's buried behind the distributor next to the firewall....Unplug and connect your VOM into the sensor half of the plug and set to ohms....
Basically it's a resistor which changes inversely to the manifold temperature..(as temp. increases resistance goes down and vice-versa)..Your trouble code is for out of range low, so your resistance is too high, anything above 15K and the sensor is junk...Start the car, as it warms up, the resistance should drop....If it doesn't get below 500 ohms, again, junk...
If the sensor tests ok, could be the harness wiring, check it ohms out to the ECM....
Basically it's a resistor which changes inversely to the manifold temperature..(as temp. increases resistance goes down and vice-versa)..Your trouble code is for out of range low, so your resistance is too high, anything above 15K and the sensor is junk...Start the car, as it warms up, the resistance should drop....If it doesn't get below 500 ohms, again, junk...
If the sensor tests ok, could be the harness wiring, check it ohms out to the ECM....
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Whoa - Didn't know that you just replaced the Plenum gaskets! Invoking the theory "that the last thing I fixed is the reason the next thing is broken" may mean that you only need to plug it back in! MAT is simply a thermistor or a device that decreases in resistance as it warms up. It gets a 5 volt reference from the ECM and as it's resistance decreases, the ECM sees a lower voltage. A 23 means it's seeing 5 volts at all times which it translates into -32 degrees and it sets the 23. If the wires were touching, that would ground the signal wire and the ECM voltage would drop to 0. The ECM sees that as 260 degrees and sets a 25. In any event, there's some probability that you either live in Nome, Alaska or the sensor wasn't plugged back in. I'd carefully check that connection.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Originally Posted by SunCr
Whoa - Didn't know that you just replaced the Plenum gaskets! Invoking the theory "that the last thing I fixed is the reason the next thing is broken" may mean that you only need to plug it back in! MAT is simply a thermistor or a device that decreases in resistance as it warms up. It gets a 5 volt reference from the ECM and as it's resistance decreases, the ECM sees a lower voltage. A 23 means it's seeing 5 volts at all times which it translates into -32 degrees and it sets the 23. If the wires were touching, that would ground the signal wire and the ECM voltage would drop to 0. The ECM sees that as 260 degrees and sets a 25. In any event, there's some probability that you either live in Nome, Alaska or the sensor wasn't plugged back in. I'd carefully check that connection.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
REPORT: I found the MAT sensor, man that thing is just about buried under everything. SunCr was right, I forgot in all my wisdom, to plug it in. I somehow managed to use some long needlenose pliers and snapped it back in. BUT, I can seem to find where the rest of the 7" connector wire goes too. It has two male prongs on the end, but no where to be plug in. Where is the other connection coming from the ecm? Also while looking I saw a single dark green wire with one male connection unplugged behind the distributor. Where in the hell does this go?
Team Owner
Joined: Jan 2003
Posts: 20,254
Likes: 221
From: Summerland B.C. Canada
Originally Posted by RRT vette
Also while looking I saw a single dark green wire with one male connection unplugged behind the distributor. Where in the hell does this go?
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Thanks Agent 86, I will have time in the morning to look a little harder. The two prong MAT sensor connector could be turned upside down under the cluster of ecm wires. As for the dark green wire, it doesn't come up very far. Not enough to go to the EGR. But I will check it too.
