An oil temperature reading of -40°F on a warm engine is a clear sign of trouble. That reading is not the weather. It is what the engine computer sees when the sensor circuit opens up. A P0198 code means the engine oil temperature sensor signal voltage is too high.
This guide explains why high voltage means a cold reading, what breaks the circuit and how to find the open.
P0198 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 98 | Specific fault | Engine Oil Temperature Sensor High |
What the P0198 Code Means
The SAE J2012 standard defines the P0198 code as Engine Oil Temperature Sensor High. The engine oil temperature (EOT) sensor is a thermistor. Its resistance rises as the oil cools, so cold oil sends a higher voltage back to the powertrain control module (PCM).
If the circuit opens, no current flows through the sensor, and the signal rises to the full 5-volt reference. The PCM reads that as the coldest value it can show, usually -40°F. Since oil cannot be that cold on a running engine, the PCM sets the code.
A broken wire, an unplugged connector, a lost ground or a sensor that failed open are the usual reasons.
Symptoms of a High Oil Temperature Sensor Signal
The engine usually runs fine. Common signs of a P0198 code include:
- Check engine light on
- Oil temperature reading -40°F or missing on the scan tool
- Oil temperature gauge stuck at the bottom
- Oil life monitor that seems off
- Variable valve timing changes on some engines
Common Causes of the P0198 Code
High input faults almost always come from an open circuit. These are the usual causes:
- Unplugged or loose connector. Often left loose after an oil change or other repair.
- Broken signal or ground wire. Wires near the oil pan can snag on debris.
- Corroded terminals. Oil and moisture add resistance.
- Failed open sensor. The thermistor breaks inside.
- Signal wire shorted to voltage. Less common, but it also reads high.
- PCM fault. Rare.
How to Diagnose the P0198 Code
You need a scan tool, a digital multimeter, a jumper wire and the wiring diagram.
Confirm the -40°F Reading
With the key on, check oil temperature in live data. A reading of -40°F, or the coldest value your tool shows, confirms an open circuit. Compare it with coolant temperature to be sure.
Inspect the Connector
Find the sensor on the oil pan, filter housing or block. Make sure the connector is fully seated and locked. Look for broken wires, bent pins and oil inside the plug. Work done near the oil filter can knock it loose.
Do the Jumper Test
Unplug the sensor and briefly connect the two connector pins with a fused jumper wire. With the key on, the scan tool should now show an extremely hot value. If it does, the wiring and PCM are fine, and the sensor is open inside. If the reading stays at -40°F, trace the wiring for a break.
Repairs That Fix the P0198 Code
Fix what you found, then clear the code and confirm the reading rises as the engine warms.
- Reconnect and lock the connector if it was loose.
- Repair broken wires with solder and heat shrink tubing.
- Clean corroded terminals or replace the pigtail.
- Replace the oil temperature sensor if the jumper test passes.
Expect a little oil to drain when the sensor comes out. Have a pan ready, use a new seal and check the oil level afterward.
After the repair, watch live data during a full warm-up. The oil temperature should start close to the outside air temperature and climb steadily as you drive. If it jumps straight back to -40°F, the open is still there, so recheck the connector, the ground and any splices in the wiring.
What Repairs Cost
Most of these repairs are cheap. This table shows typical ranges.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Reconnecting a loose connector | $0 | $50 to $100 |
| Wiring repair | $10 to $30 | $100 to $300 |
| Pigtail replacement | $15 to $40 | $100 to $200 |
| Oil temperature sensor | $20 to $80 | $100 to $250 |
Is It Safe to Drive with a P0198 Code?
Usually, yes. The PCM switches to coolant temperature or a default value, and the engine keeps running normally.
The engine loses its oil overheat protection, though, so avoid towing or hard driving until it is fixed. Watch the coolant gauge, and check the oil level if you suspect a leak around the sensor.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0194 | Fuel Rail Pressure Sensor Circuit Intermittent |
| P0195 | Engine Oil Temperature Sensor Malfunction |
| P0196 | Engine Oil Temperature Sensor Range/Performance |
| P0197 | Engine Oil Temperature Sensor Low |
| P0199 | Engine Oil Temperature Sensor Intermittent |
Restoring a Valid Oil Temperature Reading
A P0198 code means the oil temperature sensor voltage is too high, which the PCM reads as extremely cold oil. Loose connectors, broken wires and failed sensors cause most cases.
- Look for a -40°F reading in live data.
- Check that the connector is fully seated.
- Jumper the connector to test the wiring.
- Replace the sensor if the wiring passes.
Frequently Asked Questions
Why Does My Oil Temperature Read -40°F?
That is the lowest value most PCMs can show. When the sensor circuit is open, the signal rises to the full reference voltage, and the PCM reads the coldest temperature possible. It points to a disconnected sensor, broken wire or open thermistor, not cold oil.
Can an Oil Change Cause the P0198 Code?
Yes. On engines where the sensor sits near the oil filter or drain plug, the connector can be bumped loose or a wire damaged during service. If the code appeared right after an oil change, check the sensor connector first.
Will the P0198 Code Affect Fuel Economy?
Usually very little. Most PCMs use coolant temperature for fuel control. On some engines, the PCM limits variable valve timing when oil temperature is unknown, which can slightly reduce power and economy until the fault is repaired.
Is the P0198 Code the Opposite of P0197?
Yes. P0197 means the voltage is too low, usually from a short to ground, and shows a very hot reading. The P0198 code means the voltage is too high, usually from an open circuit, and shows a very cold reading. The same sensor and wiring are involved.