Fix P0198: Mechanic’s solutions that WORKED

The P0198: Engine Oil Temperature Sensor High signifies a high signal from the engine oil temperature (EOT) sensor. This sensor is crucial for calculating fuel quality, injection timing, and other engine parameters. The EOT sensor’s signal voltage decreases as the engine oil temperature increases.

P0198 quick overview

Meaning	P0198: Engine Oil Temperature Sensor High
Is it serious?	This code is of moderate to severe severity. While it may not lead to immediate vehicle breakdown, it can indicate potential engine overheating issues and should be addressed promptly.
Possible causes	– Faulty engine oil temperature sensor – Wiring problems in the sensor circuit – Faulty PCM (Powertrain Control Module)
How to diagnose?	Visual Inspection: Begin by inspecting the engine oil temperature sensor and its wiring for loose connections or damage. Repair as necessary, and clear the code to check if it returns. Check for TSBs: Look for technical service bulletins related to this issue that may provide guidance on specific makes/models. Test Cooling System: Ensure that the engine reaches and maintains the correct operating temperature. Overheating or slow warm-up may indicate a cooling system problem. Preliminary Circuit Testing: Use a scan tool to monitor EOT sensor data. Disconnect the sensor; the scan tool value should drop. Connect a jumper wire; if the scan tool displays a high temperature, the connections are likely sound, and the issue may be the sensor itself. Test the Sensor: Measure the resistance between the sensor terminals using a digital multimeter. Start the engine and observe the resistance values as the engine warms up. If resistance doesn’t decrease with increasing engine temperature, replace the sensor. Check the Circuit: Reference Voltage: Check for a 5-volt reference from the PCM at one of the sensor terminals with the ignition on. If no reference is present, test for an open circuit between the PCM and sensor. Ground Signal: Test the ground circuit between the sensor and PCM using the ohms setting on the multimeter. An OL reading indicates an open circuit. PCM Ground: Ensure that the PCM has a good ground connection to the chassis ground.

How to fix P0198 – learn from mechanics

I looked into P0198 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Dodge

Vehicle Information: 2006 Dodge Charger R/T 5.7L

Issue + Repair History: The car was towed in due to a no-start condition, previously towed from three different repair shops. Upon diagnosis, it was found that both 5-volt supply wires from the ECU had only 0.32 volts. The vehicle had been previously towed from three other repair shops, and the harness on the engine was cut open.

Other OBD2 Codes + Test Results:
P0652 – Sensor reference voltage 2 low
P0642 – Sensor reference voltage 1 low
P2127 – Pedal sensor circuit 2 low
P2122 – Pedal sensor circuit 1 low
P0118 – Coolant temp circuit high
P0198 – Oil temp sensor circuit high

Suggestions to Fix Issue:

• Unplug sensors one at a time to check if voltage returns to normal, as a bad sensor might be grounding the voltage.
• Investigate commonalities between the two temperature sensors.
• Check all power and grounds to the PCM and inspect fuses.
• Cut the two 5V wires coming out of the PCM and see if the 5V returns; if not, the PCM may be faulty.
• Disconnect various sensors and components while monitoring the 5V reference for dropouts.

Final Fix: The customer towed the car to the dealer to have a new ECU installed, which resolved the issue.

Case #2: Ford

Vehicle Information: 2000 Ford E-450 Econoline Super Duty Custom 7.3L

Issue + Repair History: The vehicle had various issues, including excessive emissions, poor idle quality, surging, cutting out, and the MIL lamp was on. Previously, the camshaft position sensor was found to be defective, and the PCM, IDM, and engine wiring harness were replaced. The HP engine oil pressure issue was also resolved. Recent issues arose after both batteries were dead, and efforts to identify and correct the problems were made. A parasitic draw of 0.6 amps was measured and could not be traced to a specific circuit. A battery cutoff switch was installed.

Other OBD2 Codes + Test Results:
PO198 – Engine Oil Temp Sensor High
PO1670 – EF Feedback Signal Not detected
PO603 – Internal Control Module KAM Error
P1210 – Injection Control Pressure High P0236 – Turbo Boost Sensor A Perform
P1211 – ICP Pressure out of spec
PO708 – Trans Range Sensor High Input
PO470 – Exhaust Pressure Sensor Fault

Suggestions to Fix Issue:

• Check for a common ground problem, as signal high codes often indicate a missing ground or an open circuit.
• Address the exhaust back pressure sensor issue and consider replacing it.
• Resolve the air charge heater circuit problem.
• Investigate the issues related to the upper alternator and the wiring associated with it.
• Check the ECT (Engine Coolant Temperature) and EOT (Engine Oil Temperature) sensors, ensuring they are functioning correctly.
• Verify the intake manifold heater operation.
• Inspect the primary crash sensor 1 feed/return circuit for open circuits.
• Reset the Keep Alive Memory (KAM) if it wasn’t done after previous engine or transmission work.

Final Fix: Several issues were identified and addressed one by one. The common ground problem in the wiring harness was resolved, and monitored PID values returned to normal. The exhaust back pressure sensor was replaced. The air charge heater circuit was repaired. An upper alternator was replaced, and wiring corrections were made. The idle charge voltage improved to 14.4 VDC, and the battery drain issue was resolved. Additional issues with the ECT and EOT sensors, as well as the primary crash sensor 1 feed/return circuit, were also addressed.

Case #3: Volkswagen

Vehicle Information:

• Year: 2016
• Make: Volkswagen
• Model: GTI
• Engine: 2.0L

Issue: The vehicle’s MIL (Malfunction Indicator Lamp) is illuminated. The issue is related to the oil level sensor and its connector. The sensor and connector were both replaced, but faults persist.

Diagnostic Steps Taken:

• The oil level sensor connector and sensor were replaced due to damage caused by the oil pan being lowered.
• A comparison of readings at the sensor was performed between the technician’s personal vehicle (MK7 Golf R) and the customer’s vehicle.
• Pin 1 had battery voltage, pin 2 had ground, which matched in both vehicles.
• Pin 3 showed differences: The technician’s vehicle had voltage fluctuating between 3 and 6 volts with the engine running, while the customer’s vehicle had a steady 12.8V at pin 3.
• Even with the sensor unplugged, the customer’s vehicle still had 12.8V on pin 3.

Possible Solutions and Additional Information:

• The technician suspected that the wire connected to pin 3 goes to the PCM (Powertrain Control Module).
• It was suggested to check the instrument cluster for a reset option related to the oil level sensor.
• The ECM (Engine Control Module) may have a short inside, causing the constant voltage on pin 3.
• The technician did not have access to Alldata for wiring diagrams and mentioned plans to check for the same color wire at the DME (Digital Motor Electronics) and proceed from there.
• The owner may need to take the vehicle to a dealer for ECM replacement and programming.

Additional Comments:

• The technician acknowledged that this issue might be more straightforward with access to the proper wiring diagrams.

Final Update: The technician found no reset option related to the oil level sensor in the instrument cluster. They attempted a hard reset but with no change in results. Cutting the wire at the ECM’s pin 83 still resulted in 12.8 volts coming out of the ECM. It was concluded that the ECM might be internally shorted. The owner was advised to take the vehicle to the dealer for ECM replacement, as the technician lacked the equipment for programming the new ECM

Case #4: Ford

Vehicle Information:

• Year: 1999
• Make: Ford
• Model: Econoline Super Duty Custom
• Engine: 7.3L

Issue: The vehicle is experiencing a stumble or misfire under acceleration, which is similar to a secondary breakdown. The technician performed various diagnostic tests and repairs but has not resolved the issue.

Diagnostic Steps Taken:

• Checked for diagnostic trouble codes (DTCs) using KOEO (Key On Engine Off) and KOER (Key On Engine Running) tests, and found no codes.
• Cylinder #6 failed the cylinder contribution test.
• Replaced the #6 injector, but the problem persisted.
• Checked under the valve covers for loose connections and installed wedge kits.
• Swapped out the IDM (Injector Driver Module) with a known good one, but the issue remained.
• Checked fuel pressure at the pump and found it to be 84-85 psi, which the technician considered high but wasn’t sure if it could cause a misfire.
• Checked for restrictions in the fuel return line and found none.
• Checked the fuel filter, which was very dirty and replaced it.
• Inspected the cam sensor and wiring, finding no issues.
• Checked oil reservoir level, which was okay.
• Observed that the EBP (Exhaust Back Pressure) reading didn’t fluctuate much under load (3-5 psi), and idle ICP (Injection Control Pressure) was 520 psi.

Possible Solutions and Additional Information:

• The high fuel pressure may be a concern, and it was suggested to check the fuel pressure regulator and related components for issues.
• Performing a cylinder buzz test with the engine cold and listening for muffled injector buzzes may help identify problems with the hydraulic plungers on the injectors.
• Running a cylinder contribution test and observing the exhaust for blue smoke can help identify a misfiring cylinder.
• The compression of cylinder #6 was mentioned as 410 psi.
• It was advised to check for bent rods from hydraulic lock.
• Checking the fuel filter bowl for dirt and restrictions and replacing the fuel regulator were recommended.
• The EBP sensor and tube should be checked for carbon buildup.
• Ensuring that the EBP sensor goes back to atmosphere when the engine is turned off was advised.

Additional Comments:

• The technician mentioned that the vehicle was a fleet vehicle and would return to work until further diagnostic steps could be taken.

Final Update: The vehicle did not return to the shop for further diagnosis, as it was part of a fleet. The technician expressed gratitude for the responses and mentioned that they would provide a further update when the vehicle became available for further inspection

Case #5: Ford

Vehicle Information:

• Year: 1995
• Make: Ford
• Model: F-350 Special
• Engine: 7.3L Diesel

Issue: The 1995 Ford F-350 with a 7.3L diesel engine is experiencing a hard start issue. The Engine Oil Temperature (EOT) sensor readings are abnormally high, indicating 270 degrees when cold and dropping to 230 degrees when hot. Despite various diagnostic efforts, the problem remains unresolved.

Diagnostic Steps Taken:

• The EOT sensor was replaced with a known good one from another truck, and the old sensor worked fine in the other truck.
• The resistance of the wires between the sensor and the PCM (Powertrain Control Module) was checked and found to be within acceptable limits.
• The voltage at pin 38 and pin 91 at the PCM connector was measured, with both pins showing appropriate values.
• The PCM was sent out for repair but returned with no issues found.
• The PCM was swapped between this truck and another, and the EOT readings were normal in the other truck.
• The Glow Plug Relay and glow plugs were checked.
• The possibility of a faulty connector or terminal connection was considered.

Possible Solutions and Additional Information:

• One suggestion was to replace the terminals at both the PCM connector and the sensor connector, as higher resistance at the PCM could result in high temp readings.
• Another recommendation was to perform a load test on the wire by unhooking both ends and jumpering B+ to one end, then jumpering the other end to a headlight bulb and grounding it. If the bulb lights brightly, the wire should be fine.
• It was mentioned that the resistance test with an ohmmeter may not detect issues if only one strand of wire is connected.
• Checking for shorts to ground or power with both the sensor and PCM disconnected was advised.
• The Glow Plug Relay and glow plugs were suggested as possible contributors to the issue.
• A booster pack was recommended for a quick test to ensure it’s not a battery-related problem causing the hard start.

Final Update: The technician did not provide a final resolution to the issue, and various suggestions were made, including checking wire connections, glow plugs, and potential battery-related problems. The problem with abnormally high EOT readings when cold remains unresolved

Advice	Diagnostic Step
Check and Replace Sensor Terminals	Replace the terminals at both PCM and EOT sensor connectors, as high resistance at the PCM can result in erroneous sensor readings and trigger P0198.
Load Test Wire for Resistance	Unhook both ends of the wire in question, jumper B+ to one end, jumper the other end to a headlight bulb, and jumper the other terminal of the bulb to ground. Check if the bulb lights up brightly to ensure the wire’s integrity. This helps ensure proper sensor signal transmission.
Check for Shorted Wires	Check for shorts to ground or power with both the sensor and PCM disconnected to eliminate potential wiring issues causing the high input signal.
Verify Glow Plug Relay and Glow Plugs	Ensure that the glow plug relay and glow plugs are in working order, as they can affect the starting process and indirectly impact sensor readings.
Consider Battery-Related Problems	Connect a booster pack as a quick test to rule out battery-related problems. Insufficient cranking power can potentially disrupt sensor data and trigger P0198.