1) What the TPS is and how it fits into the system (theory)
- The TPS (throttle position sensor) is a variable-voltage sender mounted on the throttle shaft. It tells the engine control unit (ECU) the angular position of the throttle (butterfly) so the ECU can calculate air demand.
- Most factory TPS units are a three‑wire potentiometer: 5 V reference (from ECU), ground (ECU ground), and signal (variable 0.4–4.5 V typically). The ECU reads the signal voltage and maps it to throttle angle. Some systems also have a separate idle switch contact.
- The ECU uses TPS input for fueling, idle control, EGR/boost control, torque management and limp-mode decisions. A faulty or noisy TPS produces wrong/unstable voltage, causing incorrect fueling, surging, hesitation, hard idle, and/or diagnostic trouble codes (P0120–P0124 or Nissan equivalents).

2) Preparatory safety and access (in order)
- Park on level surface, set parking brake, engine off, key out. Wear eye protection and avoid loose clothing.
- Locate the throttle body on the intake manifold and the TPS on the throttle shaft. On the YD22DDTi this is the throttle body on the intake side; the TPS is bolted to the throttle body where the throttle spindle exits.
- Disconnect negative battery terminal only if you will remove electrical connectors or replace the sensor (some testing requires battery on, so reconnect when measuring live signals).

3) Visual/wiring quick checks (first, before electrical testing)
- Inspect connector and wiring for corrosion, bent pins, chafing, pin crush or melted insulation. Wiggle the harness while watching for ECU/engine response or check light flicker.
- Clean connector with electrical contact cleaner if corroded. Repair chafed wires (solder + heatshrink or proper terminals) rather than relying on tape.

4) Static multimeter tests (order)
- Reconnect battery. Backprobe the TPS connector (or use breakout harness). Identify three wires: 5 V reference, signal, ground. Use wiring diagram or confirm by measurements:
a) With ignition ON (engine off), measure between suspected reference wire and chassis ground — should be ~5.0 V (some ECUs 4.8–5.2 V).
b) Measure between suspected ground wire and chassis ground — ~0 V (very low).
c) Measure signal wire voltage vs ground with throttle closed: typical ~0.4–1.0 V (many Nissans ~0.5 V at closed). Slowly open throttle by hand and watch voltage: it should increase smoothly and monotonically to ~4.0–4.5 V at wide open. No abrupt jumps, dead sections or noise.
- If reference voltage is missing: trace back to ECU or fuse. If signal stuck at a value or pegged to 0 V or reference, suspect short to ground or +5 V, or internal failure.

5) Dynamic/oscilloscope test (deeper theory/precision)
- With an oscilloscope, monitor the signal while you slowly open and close the throttle. A good TPS gives a smooth trace with no steps or noise spikes. Noise, discontinuities or micro‑openings indicate worn potentiometer tracks or poor internal contact — intermittent resistance causes voltage jumps and ECU confusion.
- Also check for slow voltage drift or hysteresis (signal not returning to the same voltage when throttle returns) — indicates mechanical or internal wear.

6) Resistance test (if you remove the TPS)
- With sensor unplugged and powered off, measure resistance across the end terminals (across the potentiometer). Rotate the throttle slowly: resistance should change smoothly and continuously. Any sudden jumps or infinite readings indicate internal damage.

7) Decision and repair (in order)
- If wiring/connector fault: repair wiring or connector first (replace pins, solder/splice properly), then retest. A wiring repair usually fixes lost reference or intermittent signal.
- If sensor fails electrical tests (no reference? then ECU/fuse; signal noisy/jumping or wrong range? then TPS unit): replace TPS with OE or specified equivalent.
- If mechanical binding at throttle shaft or foreign matter, clean and free the shaft and recheck. Replace if shaft play causes erratic signal.

8) Replacement procedure (order, brief)
- Disconnect negative battery.
- Unplug TPS connector.
- Remove mounting screws/bolts, support throttle body if needed.
- Remove TPS, inspect throttle shaft for wear. Fit new TPS in the same orientation; tighten to spec (hand‑tight + small torque; avoid over‑tightening).
- Reconnect connector and battery.

9) Post‑install checks and ECU learning
- Recheck voltages/signals exactly as in step 4 — closed throttle ~0.4–1 V, smooth ramp to ~4.5 V.
- Start engine and observe idle and throttle response. Clear stored codes and recheck. Some ECUs perform automatic TPS adaptation; if specified by Nissan service manual, perform the TPS idle/learn procedure (usually ignition on/off cycles or specific idle conditions). If unsure, clear codes and let the ECU relearn during normal driving.

10) How the repair fixes the fault (concise theory)
- If the TPS was worn or dirty, the potentiometer track and wiper contact were producing intermittent or incorrect voltages. Replacing or cleaning restores a stable, linear voltage proportional to throttle angle. The ECU then gets accurate air demand input and can apply correct fuel, idle control and torque limits — eliminating surging, hesitation, wrong idle and limp-mode behavior.
- If the fault was wiring/connector corrosion, repair restores proper 5 V reference and signal continuity; removing the intermittent connection stops spurious voltage readings.
- If the problem was mechanical (throttle shaft play), replacing the TPS or repairing the shaft ensures the sensor’s mechanical input matches actual butterfly position, preventing mismatch between physical throttle and ECU reading.

11) Typical fault codes you might see
- P0120–P0124 (TPS circuit malfunction, low/high/erratic), or Nissan-specific equivalents. After repair, confirm codes are cleared and do not return.

Safety and final notes (short)
- Don’t force connectors or overtighten. Keep the reference voltage measurement to ignition ON (engine off) unless instructed otherwise. Use backprobing; do not damage connector pins.

End.
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