Transmission Mechanical Failure - First And Second
Your transmission's first and second gears have suffered internal mechanical damage, like worn clutch plates or broken gear teeth. The transmission is physically broken and cannot shift properly between these gears.
The ECM monitors transmission fluid pressure, shift solenoid response times, and gear ratio changes via the input/output speed sensors. When actual gear ratios don't match commanded shifts or pressure spikes occur abnormally, the ECU detects mechanical failure.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Gear Ratio (1st/2nd) | Within 5% of expected ratio | Deviation >10% or no ratio change detected |
| Transmission Fluid Pressure | 200-350 PSI during shift | Pressure spikes >400 PSI or fails to build |
Code P1790 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1790 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.