Code Definition

P1800 — ECM to Transmission Control Module engine coolant signal

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: GM

About P1800

P1800 is a manufacturer-specific powertrain diagnostic trouble code recorded for GM vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1800 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

This DTC sets when the Transmission Control Module (TCM) does not receive a valid engine coolant temperature (ECT) signal from the Engine Control Module (ECM) over the communication network, typically Class 2 or CAN bus. The TCM uses the ECT value for cold shift scheduling and torque converter clutch engagement, since shifts and lockup are delayed until the engine warms. If the signal is missing, implausible, or out of range for a calibrated period, the TCM logs this fault. The driver may notice altered shift feel, delayed or absent torque converter lockup, or the transmission defaulting to a ...

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