NH WORKMASTER 70 code 3818

Code 3818 on New Holland/CNH platforms is a Throttle Valve Actuator (TVA) control circuit open fault, per HeavyEquipmentFix's fault-code database, and the Workmaster 70 shares its FPT engine and CNH electronic architecture with the Case IH Farmall/CASE forklift ECUs that list the same code and description.

Since you already swapped the actuator and it comes right back, the actuator itself likely isn't the problem, or the new one wasn't recognized/seated correctly. Work through these:

1. Connector at the actuator - even brand new parts get this wrong: check that the harness connector is fully latched and pins aren't pushed back or bent. An open circuit code fires immediately if the ECU sees no continuity at key-on, so a half-seated connector reproduces the code instantly even with a good part.
2. Harness continuity to the ECU - back-probe the TVA harness connector with the key on and check for the actuator's supply voltage and signal continuity per the wiring diagram. A chafed or broken wire between the actuator and ECU (common near the engine harness routing by the turbo, heat damage) throws this exact open-circuit code regardless of actuator condition.
3. Wrong part or non-calibrated actuator - some CNH throttle actuators need a matching part number or a software "teach" step after replacement (done through Case IH/New Holland Electronic Service Tools). If the new actuator's internal resistance/stroke doesn't match what the ECU expects, it can log an open/circuit fault even though it's electrically connected.
4. ECU pin or connector corrosion at the ECU side - less common but worth a visual check if 1-3 come back clean.

Start with the connector reseat and a continuity check, since that reproduces the "immediate return" symptom most often. If wiring and connectors check out clean, the actuator likely needs ECU calibration via New Holland Electronic Service Tools (EST), which isn't a DIY step without the dealer software.

If you can post whether the code returns before or after you actually try to rev the engine, that narrows it further.