Two sensors: the PCM CTS (engine management) and the gauge sender. They're separate. Replace both when they get old — cheap insurance.
The 4.0L has two coolant temp sensors, often confused:
1. PCM Coolant Temperature Sensor (CTS): two-wire sensor that the PCM uses for fuel and timing. Located in the thermostat housing on HO trucks (91+) or front of the head on Renix. NTC thermistor; resistance drops as temp rises. A drifted CTS makes the engine run rich or lean. Mopar PN 56027873AB. Cheap ($10-20).
2. Gauge Sender: single-wire sensor that drives the dash gauge. Located in the head, near the rear, behind the thermostat housing. If the gauge reads wrong, this is the part — not the CTS. Mopar PN 56027031.
When one fails, replace both — they're cheap, manageable, and the same age. Use thread sealant (not Teflon tape — debris risk) and don't overtighten the brass body.
For diagnosis: a CTS that's stuck reading cold makes the PCM run perpetually rich (poor MPG, fouled plugs, black smoke). A CTS that's stuck reading hot leans the mixture and can cause hot-start issues. The gauge sender has a similar pattern but only affects the dash readout, not the PCM.
| Part | Vendor | Est. price |
|---|---|---|
| Mopar PCM CTS | Mopar | ~$18 |
| Mopar gauge sender | Mopar | ~$22 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.