P1118 – Manifold Absolute Temperature Circuit Low Input (Ford)

By Reinier (Contact Me)
Last Updated 2016-11-14
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1118

Table of Contents

1. What Does Code P1118 Mean?
2. What are the common causes of code P1118?
3. What are the symptoms of code P1118?
4. How do you troubleshoot code P1118?
5. Codes Related to P1118
6. Get Help with P1118

What Does Code P1118 Mean?

OBD II fault code P1118 is commonly defined by Ford as “Manifold Absolute Temperature Circuit Low Input”, and is set when the PCM (Powertrain Control Module) detects a lower than expected signal in the MAT (Manifold Absolute Temperature Sensor) control circuit. Note that the terms MAT “Manifold Absolute Temperature Sensor”, and IAT “Intake Air Temperature Sensor” are sometimes used interchangeably.

The function of the IAT sensor is to measure the temperature of the air that enters the engine. Since cold air is denser than hot air, more cold than hot air can be crammed into given volume, which means that the PCM has to adjust the fuel trim continually to ensure that the ideal air/fuel ratio (14.7 parts of air to 1 part of fuel) is maintained at all times. For instance, when the intake air is very cold and therefore relatively dense, the PCM will increase the injector pulse width and adjust other parameters to enrich the air/fuel mixture to suit the denser air, and vice versa.

In terms of operation, the IAT sensor element reacts to the temperature of the air passing over it, thereby generating a signal voltage that is directly tied to the temperature of the air passing over it at any given moment. In practice, most IAT sensors work by increasing a resistance as the air passing over it gets colder, and decreasing a resistance as the air passing over it warms up. Note that there are exceptions to this rule, meaning that some sensors are electrically opposite, however; the result is the same in all cases- all IAT sensors generate signal voltages that are based on the temperature of the air passing over them.

In practical terms, a code will be set when the input voltage, usually 5 volts for most applications, is lower than expected. Since the input, or reference voltage, has a direct bearing on how well the IAT sensor performs, a lower than expected input voltage will result in incorrect or invalid signal voltages being generated, which in turn, can cause serious drivability issues on some applications.

Note however that the first failure cycle will not necessarily trigger a warning light, or set an active code. In some cases, code P1118 will be stored as a pending code until all fault code setting parameters or conditions for a particular application have been met.

The image below shows a typical combined MAF/IAT sensor, with the green arrow indicating the IAT component of the sensor. Note that MAF (Mass Airflow) sensors do not always incorporate an IAT sensor, and many applications use a standalone IAT sensor that can be located at various points in the inlet tract. Consult the manual for the application being worked on to locate the IAP sensor.

What are the common causes of code P1118?

Common causes of code P1118 are much the same across all applications, and could include the following-

• Damaged, burnt, shorted, or corroded wiring and/or connectors
• Open circuits
• Defective IAT sensors
• Defective PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.

What are the symptoms of code P1118?

In some cases, no symptoms other than a pending or an active stored trouble code and an illuminated warning light will be present. However, on some applications the symptoms of code P1118 can include serious drivability problems, which symptoms may vary in severity between applications. Common symptoms could include-

• Hard starting when the engine is cold.
• Rough or erratic idling when the engine is cold. Idling issues could improve or disappear altogether as the engine warms up.
• Increased fuel consumption.
• Hesitation or stumbling upon acceleration when the engine is cold.
• Visible black smoke may issue from the tail pipe.
• Catalytic converters can fail if this code is not resolved.

How do you troubleshoot code P1118?

NOTE #1: Standalone IAT sensors always have only two wires, one of which is a ground, and the other a combined input/signal wire that leads to and from the PCM. In cases where the IAT sensor is incorporated into the MAF sensor, the connector will usually have four wires. One wire will be the supply, or reference voltage wire for the MAF sensor, another will be a ground wire, with the remaining two wires being signal voltage wires for the MAF and IAT sensors. However, there are exceptions to this, and some combined MAF/IAT sensors have five wires. Always refer to the manual for the application being worked on to determine the function of each wire in the connector.

NOTE #2: NOTE: On models that are known for their high oil consumption rates, such as BMW, Audi, VW, Mercedes-Benz, the accumulation of oil and/or carbon on the sensing element of the IAT sensor can interfere with the sensor’s operation. Always inspect the inlet duct and sensor elements on these applications for the presence of excessive amounts of oil and/or carbon deposits, since in many cases, simply cleaning the sensor element with an approved cleaner will resolve code P1118. Also, note that these applications generally do not respond well to aftermarket replacement sensors, so always replace sensors on these applications with OEM replacement parts.

NOTE #3: To diagnose code P1118 you will need a scanner with real-time data monitoring capability, a repair manual/wiring diagram for the application, a temperature-to resistance chart for the application, a good quality digital multimeter, and a hair dryer with which to simulate changes in air temperature.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be useful should an intermittent fault be diagnosed later on.

Step 2

Consult the manual on the correct procedure (KOER/KOEO) to establish the IAT sensor’s ground connection, and set the scanner to monitor all available data from the IAT sensor. On a cold engine, the scanner should indicate a temperature reading from the sensor that will, or should be within a few degrees of the engine coolant temperature.

Step 3

If the indicated temperature is within a few degrees of the ambient temperature, chances are that both the wiring and the sensor are functional. However, depending on the application and the nature of the problem, the sensor could read a temperature of less than -40⁰F (-38⁰C), or a temperature of 300⁰F (148⁰C), or even slightly more.

Step 4

If the scanner displays an abnormally high or low temperature, consult the manual on which temperature means what, since as stated before, some IAT sensors are electrically opposite. Nonetheless, the first step should be a resistance test of the sensor, so unplug the sensor from the system, and simply test the resistance across the two terminals on the sensor if the sensor is of the standalone type.

Compare the obtained reading with the value stated in the manual, and replace the sensor if its internal resistance does not agree with the stated value.

NOTE: If the IAT sensor forms a part of the MAF sensor, consult the manual to determine the location, function, and color-coding of each wire in the connector to avoid testing the wrong MAF sensor in this case) sensor.

Step 5

If the sensors’ internal resistance agrees with the stated value, perform a thorough visual inspection of all associated wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs as required.

Step 6

If no visible damage to wiring is found, disconnect the sensor from the PCM and perform continuity, ground, reference voltage, and resistance checks on all associated wiring. Pay particular attention the resistance of the signal wire leading to the PCM: bad connections or poor crimping of terminals can increase the resistance in this wire considerably, which increased resistance has a direct bearing on the signal voltage reaching the PCM. Make repairs, or replace wiring as required to ensure that all electrical values fall within the manufacturer’s specifications.

Step 7

One easy way to test the systems after repairs were made is to use a hair dryer to simulate changing intake air temperatures. Remove the sensor from the inlet tract, but leave all wires, as well as the code reader attached.

Clear al codes, set the scanner to display the temperature as read by the sensor, and note the value. Compare this value to the manufacturer’s temperature-to-resistance (Ω) chart; the resistance for that temperature should match the value on the chart very closely. If the temperature-to resistance values match, or agree very closely, use the hair dryer to GENTLY heat the IAT sensor’s heating element, and observe the temperature value displayed by the scanner very closely.

The temperature reading should rise relatively smoothly, but take down at least four or five readings about ten degrees or so apart as the temperature rises. Compare all the noted temperature readings to the resistance chart; all temperature values should match the resistance value for that temperature very closely. If they do not, replace the IAT sensor, but only if it is certain that all electrical values for the wiring fall within the manufacturer’s specifications.

Step 8

Repeat Step 7, but in reverse. Allow the sensor to cool down, and make similar readings and comparisons until the sensor displays the same temperature as that of the engine coolant. As with Step 7, use the scanner to obtain the coolant temperature.

Step 9

Once it is certain that all electrical values for both the sensor and its associated wiring fall within specifications, make sure all codes are cleared, and operate the vehicle for at least one complete drive cycle before rescanning the system to see if any codes return. If the code does return despite having completed the repair steps outlined above, it is likely that there is an intermittent fault present in the system.

Note that intermittent faults can be extremely challenging to find and repair, and in some cases, it might be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

Codes Related to P1118

• P1119 – Relates to – “Manifold Absolute Temperature Circuit High Input” (Ford)

NOTE: P1118 is a manufacturer specific code; some manufacturers have assigned different definitions to it. Below are some examples-

• BMW – “Mass or Volume Air Flow Circuit High Input (Bank 2)”
• Daewoo/Hyundai – “Electronic Throttle System Motor Malfunction”
• Fiat – “Diesel pressure control”
• GM – “Engine Coolant Temp. Signal Out-Of-Range – High”
• Volkswagen/Audi – “O2 Sensor Heater Circ.,Bank1 – Sensor2 Open”
• Volvo – “O2 Sensor Heater Circ.,Bank1 – Sensor2 Open”

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