Code
P0218
HUMMER
P — Powertrain
Transmission Fluid Overtemperature
Views:
UK: 24
EN: 38
RU: 29
AI status
Completed
Completed
100%
Causes
- Low transmission fluid level
- Restricted or collapsed transmission cooler lines
- Clogged transmission oil cooler (radiator cooler or auxiliary cooler)
- Faulty transmission fluid temperature sensor (TFT) or bad sensor connector/wiring
- Faulty transmission control module (TCM) or related software/firmware error
- Heavy load or sustained high-speed operation (towing, steep grades, high ambient temperature)
Symptoms
- Transmission temperature warning lamp or message on dash
- Reduced performance or limp-home mode (reduced gear availability)
- Harsh, delayed, or slipping shifts
- Burnt transmission smell or dark, burned-looking fluid
- Check Engine Light/TCM lamp set alongside P0218
- Possible fluid leaks near cooler fittings or cooler lines
What to check
- Retrieve freeze-frame data and stored DTCs with a factory-level scan tool; note TFT and engine/load conditions when code set
- Visually inspect transmission fluid level and condition (while warm, on level surface, following factory procedure)
- Check transmission fluid color and smell for burning or contamination
- Read live data: transmission fluid temperature, engine coolant temp, TCM status, vehicle speed, gear commanded/actual
- Inspect transmission cooler lines, fittings, cooler core and radiator cooler for restrictions, kinks, leaks or blockages
- Inspect wiring harness and connector to the TFT sensor and TCM for damage, corrosion or poor connection
Signal parameters
- Typical ATF operating temp: ~70–95°C (158–203°F) during normal driving (varies by model)
- Caution range: ~105°C (221°F) and above; fault threshold commonly >120°C (248°F) (manufacturer-specific)
- TFT sensor output: typically a temperature-dependent voltage/resistance (NTC style) — expect a stable, monotonic voltage change with temperature (consult factory values)
- Live-data update rate: TFT value usually updates >=1 Hz on the vehicle bus/scan tool
- Compare reported TFT vs engine coolant temp and ambient to detect sensor/reporting errors (large unrealistic deviations suggest sensor/wiring fault)
Diagnostic algorithm
- Connect a capable scan tool and read all stored transmission and engine codes, freeze-frame and live TFT values. Note conditions when code set (idle/load/speed).
- Visually inspect transmission fluid level and condition. If fluid is low or severely degraded, top/replace per service procedure, clear codes and retest. Do not overfill.
- Inspect external transmission cooler, radiator cooler section, cooler lines and fittings for blockages, damage, kinked lines or internal collapse. Repair/replace as required.
- Verify engine cooling fans, fan clutch (if fitted) and airflow across cooler are functioning correctly. Repair cooling system faults.
- With scan tool, monitor TFT while warming to operating temp and during a drive cycle. If TFT reads excessively high under normal load, proceed.
- Test TFT sensor and wiring: check connector condition, backprobe for sensor voltage/resistance compared to factory specification and verify proper change with temperature (sensor removed and heated/cooled if needed). Repair wiring or replace sensor if out of spec.
- If sensor and cooling circuit check good but TFT still high, perform transmission pressure/flow checks and inspect for internal overheating causes (slipping clutches, torque converter lockup problems).
- If no mechanical fault found, consider TCM calibration or hardware fault—verify TCM power/ground and communications; update software if factory bulletin applies;replace TCM only after confirming sensor/ mechanical-cooling system integrity.
- After repairs, clear codes and perform an extended road test with live data to confirm TFT remains within normal operating range under expected loads.
Likely causes
- Low fluid level or severely degraded/contaminated fluid
- Restricted/clogged or leaking transmission cooler or cooler lines
- Faulty TFT sensor or wiring (open/short/intermittent)
- Cooling system fault reducing cooler capacity (fan, radiator, or auxiliary cooler obstruction)
Fault status
Status
Transmission fluid temperature has exceeded the allowed threshold. Immediate inspection of fluid level/condition and the transmission cooling system is recommended to prevent damage.
Repair difficulty: Medium
Diagnostic time: 1.0 - 4.0 hours
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