P1290

What Does Code P1290 Mean?

DTC P1290 signifies that the Engine Control Module (ECM), also commonly referred to as the Powertrain Control Module (PCM), has detected an input signal from the Cylinder Head Temperature (CHT) sensor that is below its calibrated operational range or a predetermined low voltage threshold. The CHT sensor is typically a Negative Temperature Coefficient (NTC) thermistor, meaning its electrical resistance decreases as its temperature increases. Conversely, resistance increases as temperature decreases. The ECM supplies a reference voltage (typically 5V) to the CHT sensor, and by measuring the voltage drop across the sensor’s resistance, it infers the cylinder head’s temperature.

A “low input” condition for an NTC CHT sensor typically indicates an abnormally high resistance or an open circuit within the sensor itself or its wiring. From the ECM’s perspective, an open circuit (infinite resistance) would result in the maximum voltage drop across the sensor, meaning the voltage signal returning to the ECM would be at or near ground potential (0V), interpreted as an extremely low temperature (e.g., -40°F / -40°C). This can also occur if the signal wire is shorted to ground. The ECM uses CHT data for critical engine management strategies including fuel mixture enrichment, ignition timing adjustments, idle speed control, and cooling fan activation. When P1290 is set, the ECM usually enters a fail-safe mode, often defaulting to a very cold engine condition, which leads to rich fueling and potentially altered cooling system behavior to prevent perceived overheating, even if the actual temperature is within limits.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination (Check Engine Light).
• Hard starting, particularly when the engine is cold or at ambient temperatures, due to an excessively rich fuel mixture.
• Rough idle or poor engine performance, including hesitation or lack of power, stemming from incorrect fuel trim and ignition timing.
• Excessive fuel consumption, as the ECM commands a richer air/fuel ratio based on the erroneous cold temperature reading.
• Black smoke from the exhaust tailpipe, indicative of a very rich running condition where unburnt fuel is expelled.
• Engine cooling fans running constantly or cycling on more frequently than normal, as the ECM may default to a “cold engine” or “fail-safe” cooling strategy when reliable temperature data is unavailable.
• Lack of engine heat for the HVAC system inside the cabin, although this is less common and more indirectly related.

What Causes the Code P1290?

• Faulty Cylinder Head Temperature (CHT) Sensor: The sensor itself may have an internal open circuit, short to ground, or simply drift out of its specified resistance range, leading to an incorrect voltage signal.
• Open circuit in the CHT sensor wiring: A break in the signal wire, reference voltage wire, or ground wire between the CHT sensor and the ECM/PCM.
• Short to ground in the CHT sensor wiring: The signal wire or reference wire inadvertently making contact with a chassis ground.
• Corroded or loose electrical connectors: Poor electrical contact at the CHT sensor connector or at the ECM/PCM connector pins can impede signal transmission.
• Faulty Engine Control Module (ECM/PCM): Although less common, an internal failure within the ECM’s input circuit for the CHT sensor can cause this code. This should only be considered after thoroughly verifying the sensor and wiring integrity.

How to Diagnose and Troubleshoot

Diagnosis of P1290 requires careful electrical testing and systematic elimination of potential fault points.

1. Retrieve and Document DTCs: Connect an OBD-II scanner to the vehicle’s DLC. Note all stored, pending, and historic Diagnostic Trouble Codes. Pay attention to freeze frame data, as it provides engine conditions at the moment the DTC was set.
2. Visual Inspection:
   • Locate the CHT sensor (typically threaded into a cylinder head, often between spark plugs).
   • Inspect the CHT sensor connector for corrosion, bent pins, proper seating, and any signs of physical damage.
   • Follow the wiring harness from the CHT sensor back towards the ECM. Look for chafing, bare wires, pinch points, signs of rodent damage, or any obvious breaks in the insulation.
3. OBD-II Scanner Live Data Analysis:
   • Monitor the live data stream for the CHT sensor reading. If P1290 is present, the CHT sensor reading will likely be stuck at an extremely low value (e.g., -40°F / -40°C) or a fixed default value.
   • Compare the CHT reading with other available temperature sensors (e.g., Intake Air Temperature (IAT) sensor) when the engine is cold and at ambient temperature; their readings should be reasonably close.
4. Digital Multimeter (DMM) Testing at the CHT Sensor Connector (Ignition OFF, Sensor Disconnected):
   • Sensor Resistance Test: Disconnect the electrical connector from the CHT sensor. Measure the resistance across the two terminals of the CHT sensor itself. Compare this reading to manufacturer specifications (refer to a factory service manual for a resistance-to-temperature chart). For an NTC thermistor, an open circuit (infinite resistance) or a resistance value significantly higher than specified at ambient temperature indicates a faulty sensor.
   • Reference Voltage Test: With the ignition ON and engine OFF, measure the voltage between the reference voltage terminal (typically 5V) and a known good chassis ground at the harness side of the CHT sensor connector. You should measure approximately 5 Volts.
   • Ground Circuit Test: Check for continuity between the ground terminal at the harness side of the CHT sensor connector and a known good chassis ground. Resistance should be less than 5 ohms.
   • Signal Circuit Test: To verify the signal wire integrity back to the ECM, you can simulate a sensor input. With the ignition ON, engine OFF, and CHT sensor disconnected, temporarily connect a suitable resistor (e.g., 2.2 kΩ or 4.7 kΩ, depending on manufacturer specifications for a middle-range temperature) between the 5V reference wire and the signal wire at the harness connector. Observe the CHT reading on the OBD-II scanner. A plausible temperature reading indicates the wiring and ECM input are likely functional. Alternatively, shorting the signal wire to the ground wire at the harness connector should typically cause the scanner to display a very high temperature, as this simulates extremely low sensor resistance.
5. Continuity and Short to Ground Tests (Sensor to ECM):
   • If preliminary tests are inconclusive, disconnect both the CHT sensor and the ECM/PCM connectors.
   • Using a DMM, check for continuity of each wire (signal, reference, ground) from its pin at the CHT connector to its corresponding pin at the ECM connector. Resistance should be very low (< 1 ohm).
   • Check for shorts to ground and shorts to power (battery voltage) on each wire in the CHT circuit. Place one DMM lead on the wire terminal and the other on a known good chassis ground or battery positive terminal. Resistance should be infinite.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

1. Replace the Cylinder Head Temperature (CHT) Sensor: If the sensor itself tested faulty (e.g., open circuit, out-of-spec resistance), replacement is the primary solution. When installing, ensure the correct torque specification is applied and use thread sealant if specified by the manufacturer to prevent coolant leaks (some CHT sensors are dry, others contact coolant indirectly).
2. Repair or Replace Wiring Harness: If diagnostic steps reveal an open circuit, short to ground, or damaged insulation in the CHT sensor wiring, the affected section of the harness must be repaired or replaced. Utilize automotive-grade wiring, proper soldering or crimping techniques, and heat-shrink tubing for durable, weather-resistant repairs.
3. Clean or Replace Electrical Connectors: If corrosion, bent pins, or poor terminal tension are identified at either the CHT sensor connector or the ECM/PCM connector, clean the terminals using electrical contact cleaner or, if severely damaged, replace the connector shell and terminals.
4. ECM/PCM Replacement: This should be considered a last resort, only after the CHT sensor and its entire wiring circuit have been thoroughly verified as fully functional and within specifications. ECM replacement often requires specialized programming or “flashing” to the vehicle, which may need to be performed by a dealership or a specialized independent shop.

After any repair, clear the DTCs using an OBD-II scanner. Perform a comprehensive drive cycle to allow the ECM to re-evaluate the CHT sensor input and confirm that the P1290 code does not return. Continuously monitor live CHT data during the drive cycle to ensure the sensor provides accurate and plausible temperature readings.