P0127 – Intake air temperature too high

By Reinier (Contact Me)
Last Updated 2017-08-09
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0127 Mean?
2. What are the common causes of code P0127 ?
3. What are the symptoms of code P0127 ?
4. How do you troubleshoot code P0127 ?
5. Codes Related to P0127
6. Get Help with P0127

What Does Code P0127 Mean?

OBD II fault code P0127 is a generic code that is defined as “Intake air temperature too high”, and is set when the PCM (Powertrain Control Module) detects an abnormally high signal voltage from the IAT (Intake Air Temperature) sensor. Note that this code refers to a high voltage in the control circuit of the IAT sensor, and NOT to the actual temperature of the intake air.

In a fully functional system, the PCM uses input data from both the MAF (Mass Airflow) sensor and the IAT sensor to calculate fuel delivery strategies. While the MAF sensor monitors the total volume of air that enters the engine, the IAT sensor monitors the actual temperature of the intake air.

Intake air temperature plays a critical role in fuel management, since the hotter the intake air is, the less dense it becomes, meaning that in order to maintain a stoichiometric (14.7: 1) air fuel mixture, the PCM needs to continually adapt the volume of fuel delivered by the injectors to the volume of available air, whose density changes as its temperature changes.

On most applications the IAT sensor is a simple thermistor whose resistance changes as the temperature of the air that flows across it changes. In practice, the PCM delivers a 5-volt reference voltage to the sensor, which voltage changes in direct relation to the changes in the resistance of the sensing element. The changes in resistance are signalled to the PCM via a dedicated signal wire, to be interpreted by the PCM as changes in the temperature of the intake air.

Note that it is common for abnormally high signal voltages to be interpreted as impossibly high intake air temperatures. In some cases, a scanner might indicate an intake air temperature of hundreds of degrees, which is clearly impossible. Thus, when the PCM detects a signal voltage that exceeds the maximum allowable voltage, the PCM will set code P0127, and may also illuminate a warning light.

The image below shows a typical Intake Air Temperature sensor. Note though that the appearance and location of these sensors vary widely between applications and manufactures, so refer to the manual for the application to identify/ locate the IAT sensor correctly to avoid a misdiagnosis.

What are the common causes of code P0127 ?

Some common causes of P0127 could include the following-

• Damaged , burnt, shorted, disconnected, or corroded wiring and/or connectors
• Defective IAT sensor
• Major air leaks, depending on where in the inlet tract the IAT sensor is located
• On some applications, a defective MAF sensor can contribute to the setting of P0127
• Failed or failing 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 P0127 ?

Some common symptoms of code P0127 could include the following-

• Stored trouble code and possibly an illuminated warning light
• Other driveability codes could be present as well
• Idling may be rough, or the idling speed may fluctuate
• Engine may hesitate, surge, or stall upon acceleration when the engine is cold
• Fuel consumption may increase considerably
• Most applications will experience some degree of power loss

How do you troubleshoot code P0127 ?

NOTE: Apart from a scanner/code reader, an infrared-based thermometer and a hair dryer will be most helpful in diagnosing this code.

Step 1

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

NOTE: If any other codes are present, refer to the manual to determine their relationship to P0127. Note however that codes that were stored after P0127 have likely set as a result of P0127, but in all cases where other codes precede P0127 these codes must be resolved before an attempt is made to diagnose P0127. Failure to do will almost certainly result in a misdiagnosis, wasted time, and the unnecessary replacement of parts and components.

Step 2

Refer to the manual to locate the IAT sensor. Also determine the color-coding and function of each wire in the sensor connector/harness.

NOTE: Check that the connector is fastened securely, since it is common for this connector to be left undone after routine maintenance.

Step 3

Perform a thorough visual inspection of the sensor connector and wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs or replace wiring as required.

Step 4

If no visible damage to wiring is found, prepare to perform reference voltage, resistance, ground integrity and continuity checks as per the instructions in the manual on all associated wiring. Be sure however to disconnect all wiring from the PCM to prevent damaging the controller during resistance testing. Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring as required if deviations are found.

NOTE #1: Pay particular attention to the reference voltage and signal circuits. If no reference voltage is present, refer to the manual top determine the correct procedure to follow to see if the PCM is actually delivering a reference voltage. If it does not, replace the PCM, but bear in mind that if the reference voltage circuits in the PCM are defective, several other sensors will be affected as well.

NOTE #2: Abnormally high signal voltages are commonly caused by short circuits to battery positive between the signal wire and other “live” wires. Be sure therefore to check the entire length of the signal wire for signs of short circuits.

Step 4

If all electrical values in the circuit check out, remove the IAT sensor from the inlet tract but leave the connector connected. Use the hair dryer to GENTLY heat the sensor while monitoring the sensor’s output on the scanner. Depending on the application, the voltage will change either up or down as the sensor is heated. Use the infrared thermometer to obtain several AACCURATE readings of the temperature of the sensor, and compare these to the manufacturers’ temperature-to-voltage chart.

Replace the IAT sensor with an OEM replacement if any obtained temperature reading does not agree with the values specified by the manufacturer.

Step 5

Clear all codes after all repairs are complete, and operate the vehicle for at least one complete drive cycle to see if the code returns. In the unlikely event that this or any other codes return, suspect an intermittent fault in either the wiring or the sensor itself.

Bear in mind that many aftermarket sensors do not provide the same level of accuracy or sensitivity that OEM replacements do. Thus, if the IAT sensor is replaced with an aftermarket unit, replace it with a new OEM-standard sensor before attempting to locate an intermittent fault. If however the fault persists despite an OEM sensor being fitted, the better option is to refer the vehicle to a competent repair facility for professional diagnosis and repair.

Codes Related to P0127

While there are no known codes that are directly related to P0127 – “Intake air temperature too high”, several MAF related issues can contribute to the setting of P0127. Note however that the contributing MAF related codes largely depend on the make and model.

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