Summary (one line)
- The oil temperature sensor is a thermistor that tells the ECU/gauge oil temperature by changing resistance; replacement restores the correct resistance/voltage signal and fixes bad readings or codes caused by a failed sensor, wiring or seals.

Theory first (concise)
- Type: most oil temp sensors are thermistors (usually NTC) — resistance falls as temperature rises. The ECU supplies a reference (voltage or ground) and measures the resulting voltage drop to compute temperature.
- Circuit failure modes: open circuit (infinite resistance) → no or erratic reading; short to ground/voltage → pegged reading; corrosion/poor connector → intermittent/erroneous readings; leaking sensor or damaged seal → oil loss and contamination of connector.
- Effect on vehicle: bad signal can throw diagnostic trouble codes, show wrong gauge readings, prevent correct ECU decisions (fan control, cold compensation) and cause oil leaks if the sensor or seal fails.

Ordered repair + diagnostic steps (do these in order)
1. Safety and preparation
- Work with engine OFF and cool. Park on level ground, set parking brake. Use jack stands if lifting.
- Tools: multimeter, appropriate deep socket or sensor wrench, torque wrench, new sensor (OEM spec), new crush washer/seal, oil catch pan, rags, dielectric grease, small mirror/torch.
- Recommended: disconnect negative battery terminal to avoid shorts while handling electrical connectors.

2. Locate the sensor
- On the 5L-E the oil temperature sender is in the block/filter housing area (consult factory manual for exact location). Remove any engine covers/undertray that block access.

3. Baseline diagnostics BEFORE removing
- Visually inspect connector and wiring for corrosion, bent pins, damaged insulation, oil contamination.
- Back-probe the sensor connector with a multimeter: measure resistance across terminals or the voltage referenced by the ECU (with ignition on, engine off). Compare to factory spec table for temperature vs resistance/voltage. If readings are out of spec, the sensor is suspect.
- Check continuity of wiring to the ECU/ground to rule out harness faults.

4. Prepare for oil drip
- Place catch pan under sensor location. You generally do not need to drain the full engine oil — a small stream will escape when the sender is removed.

5. Remove the old sensor
- Disconnect the electrical connector and any retaining clip.
- Unscrew the sensor with the correct wrench/socket. Expect some oil to drain; contain it.
- Inspect the sensor threads and port for debris and inspect the old crush washer/seal.

6. Inspect harness and port
- Check mating connector pins for corrosion and measure resistance of harness to ECU pin to confirm wiring integrity.
- Clean the port threads and mating surfaces. Remove oil/dirt — do not introduce debris into the oil gallery.

7. Fit the new sensor
- Use the correct replacement sensor and a new crush washer or seal per parts manual. Do not reuse the old washer.
- If the manufacturer requires thread sealant, use the recommended product; otherwise do NOT use excessive Teflon tape (it can shear off). A light smear of engine-safe anti-seize on threads is acceptable if manual allows.
- Screw in by hand, seat the washer, then tighten to factory torque specification (refer to Toyota service manual). If the manual is unavailable, tighten snugly with an appropriate final torque (typical temp-sensor/tube fittings are in the ~20–30 N·m range — check manual for exact figure).

8. Reconnect and seal
- Reconnect electrical connector; apply a small dab of dielectric grease to pins to prevent corrosion.
- Reinstall any removed covers/undertrays.

9. Reconnect battery and clear codes
- Reconnect negative battery terminal.
- Clear any stored codes with a scanner. If you don’t have a scanner, codes may clear after several drive cycles, but using a scanner is preferred.

10. Functional check
- Start engine, check for oil leaks around the new sensor.
- Monitor oil temperature reading (gauge or ECU live data). With a cold engine the reading should be low, then rise smoothly as engine warms. No erratic jumps or pegging.
- If possible, compare live sensor voltage/resistance to expected values at measured oil temperature (factory chart).

How the repair fixes the fault (concise)
- Replacing the sensor restores a correct thermistor element (resistance vs temperature). That fixes open/short or out-of-spec resistance that produces wrong voltages to the ECU/gauge.
- Replacing the connector or cleaning pins fixes intermittent/poor contacts that cause erratic signals.
- Installing a new crush washer/seal eliminates leaks that could cause oil loss or contaminate the connector and produce electrical faults.
- After installation and clearing codes, the ECU receives accurate temperature input so gauge readings, engine controls (fan, fueling corrections) and diagnostics behave normally.

Quick troubleshooting reminders (bullet)
- If the new sensor behaves the same, re-check wiring continuity and ECU input pin — the harness/ECU may be at fault.
- Always use OEM or exact-spec replacement sensors and seals.
- Torque and seal method should follow Toyota service manual to avoid leaks or thread damage.

End.
rteeqp73

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