Here's what you should do:
* Verify the Code: While you mention code 172, double-check the code using a reliable OBD-I scanner or a method specific to your year's F-150's diagnostic system (checking the flashing codes from the ALDL connector). Make sure it's truly a 172 and not a similar code.
* Inspect the TPS: Locate the TPS on the throttle body. It's usually a small sensor mounted on the side. Visually inspect it for any damage, loose wiring, or corrosion.
* Check the TPS Wiring: Carefully examine the wiring harness connected to the TPS for any broken, frayed, or corroded wires. Pay close attention to the connector itself.
* Test the TPS Voltage: This requires a multimeter. With the engine off, you'll need to check the voltage at the TPS connector, comparing the readings to the specifications in your Ford repair manual (very important!). You'll likely need to check for voltage at different throttle positions (closed and fully open). Incorrect voltages strongly suggest a faulty TPS.
* Clean the Throttle Body: A dirty throttle body can affect the TPS readings. Clean it thoroughly with a throttle body cleaner, following the instructions carefully. Make sure the throttle plate moves freely after cleaning.
* Replace the TPS: If the inspection, wiring check, and voltage test all point to a faulty TPS, replacement is the most likely solution. Ensure you get a replacement TPS that's specifically designed for your 1994 Ford F-150 5.0L engine. After replacement, clear the trouble codes.
Important Considerations:
* Repair Manual: A factory service manual for your specific year and model is invaluable. It will provide detailed diagnostic procedures, wiring diagrams, and specifications.
* Professional Diagnosis: If you're not comfortable working on your vehicle's electrical system, take it to a qualified mechanic for diagnosis and repair. Improperly diagnosing or repairing electrical issues can lead to further damage.
* OBD-I Limitations: Remember that OBD-I systems are less sophisticated than modern OBD-II systems. The diagnostic codes are less specific, and you may need to perform more troubleshooting steps to pinpoint the exact problem.
The check engine light coming on *after* the engine warms up suggests the problem might be related to heat affecting a component, like a sensor's internal resistance changing with temperature. This points even more strongly towards the TPS as a likely culprit.
