Intake Air Temperature Sensor 1 Circuit Low

What P0112 means

The intake air temperature (IAT) sensor is integrated into the mass air flow (MAF) meter housing and uses an NTC thermistor: resistance falls as temperature rises, pulling the ECM's 5 V reference (supplied on the THA terminal) progressively lower. The normal operating voltage range is 0.18 V to 4.91 V, corresponding to intake temperatures between -55 °C and 128 °C. P0112 is stored as a 1-trip fault when the THA signal drops below 0.18 V for 0.5 seconds or more. At that point the ECM calculates a temperature above 128 °C — physically unreachable in a functioning intake — and treats the reading as a short circuit. During fail-safe mode the ECM substitutes a fixed IAT of 20 °C for all fuel and ignition calculations until the signal returns to range. Battery voltage must be above 8 V and the starter must be off for the monitor to run.

Symptoms

• Check engine light on immediately — 1-trip MIL illumination
• Scan tool displays intake air temperature above 128 °C or stuck at an implausibly high fixed value
• Possible lean fuel correction or ignition timing changes in fail-safe if actual intake temperature differs from the 20 °C substitute
• No noticeable drivability change in mild ambient conditions — fail-safe value of 20 °C is a reasonable approximation at room temperature
• Slightly degraded fuel economy in very cold climates where 20 °C substitute causes under-enrichment at cold start

Common causes

• Short circuit in the THA (IAT signal) wire to ground — most common direct cause of sub-0.18 V reading
• Failed IAT sensor element within the MAF meter housing — internal short in the thermistor producing a collapsed resistance
• Liquid contamination inside the MAF meter connector — water ingress across signal and ground pins can create a conductive path
• Chafed harness between the MAF meter and ECM where the THA wire contacts a ground point
• Damaged ECM connector with signal-to-ground short at the THA input pin

Severity & driving advice

Severity: low — Fail-safe substitutes 20 ?C IAT ? drivable in most conditions. Cold-climate cold-start calibration slightly degraded. No immediate mechanical risk.

Can I drive? Driveable short-term. Repair within 1 week.

Diagnostic approach

1. Read live IAT in scan tool Data List — With ignition on, check the intake air temperature reading. An IAT display above 128 °C (262 °F) confirms the low-voltage short-circuit condition responsible for P0112. A reading at -40 °C would instead suggest an open circuit (P0113). Do not confuse the two — they require opposite diagnostic paths.
2. Disconnect the MAF meter connector and re-read scan tool IAT — Unplug the MAF meter connector. The IAT signal input should now float to the ECM pull-up voltage, and the scan tool should display -40 °C (open circuit with the sensor disconnected). If the scan tool still reads above 128 °C after disconnecting the MAF meter, the short to ground is in the wire harness or at the ECM connector — not in the sensor or MAF meter itself.
3. Test the THA wire harness for short to ground — With the MAF meter connector and ECM connector both disconnected, measure resistance between the THA signal wire pin (at the MAF meter harness connector) and chassis ground. A good harness reads 10 kΩ or higher. A reading below 1 kΩ confirms a short to ground in the wiring between the MAF meter and ECM.
4. Inspect the MAF meter connector for moisture or corrosion — Open the connector and check for water deposits, green corrosion, or conductive debris bridging the THA and E2 (ground) terminals. Blow out with compressed air, clean with electrical contact cleaner, and allow to dry fully before reconnecting. Liquid contamination inside connectors is a common but overlooked cause of temporary low-voltage IAT faults.
5. Verify ECM connector integrity on the THA pin — Inspect the ECM-side connector for the THA pin condition. With both the MAF meter connector and ECM connector unplugged, measure resistance from the THA wire at the ECM end to chassis ground. Should be 10 kΩ or higher. If below 1 kΩ, the short is between the MAF meter connector and the ECM connector itself.
6. Replace the MAF meter if all wiring checks are clean — If the harness and connectors test within spec and the fault traces back to the MAF meter itself, replace the MAF meter assembly (the IAT sensor is not separately serviceable — it is integrated into the MAF housing on most vehicles). After replacement, perform the initialization procedure if required by your vehicle's ECM, then clear codes and verify IAT reads correctly at ambient temperature.

Make & model notes

Toyota: On Toyota vehicles using VG4SF-type hot-wire MAF meters, the IAT sensor is a separate pin inside the same multi-pin MAF connector. The connector is exposed to oil mist from the PCV system on some engines, which can deposit a conductive film across the sensor pins over time. A MAF meter cleaning with MAF cleaner spray sometimes temporarily resolves P0112 before a full replacement is needed.

Ford: Many Ford EcoBoost applications have a standalone IAT sensor in the intercooler outlet pipe (separate from the MAF meter). On these, P0112 traces to a much simpler two-wire sensor — replacement costs under $30 for the sensor itself.

GM: GM trucks and SUVs with the 5.3L or 6.2L V8 use a combined MAF/IAT sensor. A known concern is moisture accumulation inside the filter housing after rain or car washing that wicks into the MAF connector, causing temporary P0112 that clears on its own — inspect for water intrusion before replacing the MAF meter.

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