P0099 IAT Circuit Sensor 2 Intermittent
OBD-II Trouble Code Technical Description
Intake Air Temperature Sensor 2 Circuit Intermittent
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to all 1996-newer vehicles (Ford, Mazda, Mercedes Benz, etc.). Although generic, the specific repair steps may vary depending on make/model.
A stored code P0099 means that the powertrain control module (PCM) has detected an intermittent input signal from the #2 intake air temperature (IAT) sensor circuit.
The PCM uses the IAT input signal and the mass airflow (MAF) sensor input signal to calculate fuel delivery and ignition timing strategy. Since maintaining a correct air/fuel ratio (typically 14:1) is critical to engine performance and fuel efficiency, the input of the IAT sensor is very important.
The IAT sensor can be threaded directly into the intake manifold but is more commonly inserted into the air inlet pipe or air cleaner box. Some manufacturers also integrate the IAT sensor into the MAF sensor housing. At any rate, it must be positioned so that (with the engine running) ambient air, drawn into the intake manifold through the throttle body, can continuously and consistently flow across it.
The IAT sensor is usually a two-wire thermistor sensor. Sensor resistance varies according to the temperature of the air that flows across the cold wire element. Most OBD II equipped vehicles utilize a reference voltage (five-volts is normal) and a ground signal to complete the IAT sensor circuit. The varying resistance levels in the IAT sensor element cause voltage fluctuations in the input circuit. These fluctuations are interpreted by the PCM as changes in intake air temperature.
If the PCM detects a certain number of intermittent signals from the #2 IAT sensor, over a set period of time, a code P0099 will be stored and a malfunction indicator lamp may be illuminated.
Severity & Symptoms
The IAT sensor signal is used by the PCM to calculate fuel delivery strategy, therefore a code P0099 should be treated as severe.
Symptoms of a P0099 code may include:
Slightly diminished fuel efficiency Decreased engine performance (especially upon cold start) Hesitation or surge at idle or upon light acceleration Other drivability codes may be storedCauses
Possible causes for this engine code include:
Open or shorted #2 IAT sensor wiring and/or connectors Defective #2 IAT sensor Faulty MAF sensor Clogged air filter Broken intake air inlet pipeDiagnostic and Repair Procedures
A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known issue with a known fix put out by the manufacturer and can save you time and money during diagnosis.
When faced with diagnosing a code P0099, I like to have a suitable diagnostic scanner, a digital volt/ohmmeter (DVOM), an infrared thermometer, and a reliable vehicle information source (such as All Data DIY) at my disposal.
Connect the scanner to the vehicle diagnostic connector and retrieve the stored trouble codes and applicable freeze frame data. I normally write this information down in case I need it later. Clear the codes and test drive the vehicle. If the code is immediately reset, proceed with the diagnostic process.
Most professional technicians begin with a visual inspection of IAT sensor related wiring and connectors (don’t forget the air filter and the air inlet pipe). Pay careful attention to the sensor connector as it is susceptible to corrosion because of its close proximity to the battery and coolant reservoir.
If the system wiring, connectors, and components appear to be in working order, proceed by connecting the scanner to the diagnostic connector and opening the data stream. By n
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