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P0398 Code: Cylinder 1 Pressure Sensor Circuit High

P0398 Cylinder 1 pressure-sensor circuit-high diagnostic illustration showing excessive signal voltage

The P0398 OBD-II code means the engine control module has detected voltage above the permitted threshold in the Cylinder 1 pressure-sensor circuit.

On applicable diesel engines, the combustion-pressure sensor may be integrated into a specialized pressure-sensing glow plug. The sensor allows the engine controller to monitor the pressure developed inside Cylinder 1 during compression and combustion.

P0398 is an electrical circuit-high code. It does not automatically mean Cylinder 1 has excessive compression, abnormally high combustion pressure or too much fuel pressure.

Before anyone replaces an injector, removes the cylinder head or orders an engine from another zip code, the sensor circuit needs to be tested.

P0398 Quick Reference

ItemInformation
CodeP0398
DefinitionCylinder 1 Pressure Sensor Circuit High
SystemIn-cylinder combustion-pressure sensing
Typical applicationCertain modern diesel engines
Fault typeElectrical signal above its permitted threshold
Common sensor designPressure sensor integrated into a glow plug
Common causesOpen signal circuit, short to voltage, failed sensor or open low reference
Does it prove high cylinder pressure?No
Can it affect engine operation?Yes
Recommended responseDiagnose promptly
Typical repairWiring, connector, sensor or pressure-sensing glow-plug repair

What Does the P0398 Code Mean?

P0398 sets when the powertrain control module or engine control module sees Cylinder 1 pressure-sensor voltage remain above a calibrated electrical limit.

The controller expects the pressure-sensor signal to operate within a defined range and change as cylinder pressure rises and falls. If the signal remains excessively high for a specified period, the controller stores P0398 and may illuminate the Check Engine Light.

Possible electrical conditions include:

The exact voltage threshold and code-setting conditions depend on the manufacturer.

Published diesel service information identifies P0398 as Cylinder 1 Pressure Sensor Circuit High Voltage. Its diagnostic circuit table shows that an open signal circuit, a signal shorted to voltage or an open low-reference circuit can produce the high-voltage fault. Review the applicable Saab service information.

What Is a Cylinder-Pressure Sensor?

A cylinder-pressure sensor measures pressure directly inside the combustion chamber.

Traditional engine-management systems estimate combustion quality using indirect information such as:

An in-cylinder sensor gives the controller direct information about how pressure develops during compression and combustion.

Depending on the vehicle, this data may help the PCM control:

Some diesel engines integrate the pressure sensor into the glow plug. This component may be called a:

A regular glow plug does not contain the required pressure-sensing electronics and cannot replace one simply because the threads and connector look vaguely cooperative.

How a Pressure-Sensing Glow Plug Works

A pressure-sensing glow plug performs two separate jobs:

  1. Its heating element supports cold starting and combustion.
  2. Its integrated pressure sensor monitors cylinder pressure.

Combustion pressure acts on the sensing element. Internal electronics convert that force into an electrical signal that the PCM can interpret.

The sensor circuit may include:

The heating element and pressure sensor can fail independently.

A glow plug that heats correctly may still produce an invalid pressure signal. Likewise, a pressure sensor may operate while the heating element has failed.

P0398 specifically directs diagnosis toward the high pressure-sensor signal—not automatically toward the glow-plug heater circuit.

Circuit High Does Not Mean Cylinder Pressure Is High

The word “high” describes the electrical signal voltage seen by the PCM.

It does not prove that pressure inside Cylinder 1 is physically too high.

P0398: Electrical Signal High

Possible causes include:

Actual Cylinder Pressure Too High

Possible causes may involve:

P039B is the neighboring code intended for actual Cylinder 1 pressure that is too high.

Diagnosing P0398 as a mechanical overpressure problem before testing the electrical circuit is the automotive equivalent of renovating the kitchen because a light switch stopped working.

P0398 vs. Related Codes

CodeDefinitionPrimary diagnostic direction
P0395Cylinder 1 Pressure Sensor CircuitGeneral circuit malfunction
P0396Cylinder 1 Pressure Sensor Circuit Range/PerformanceSignal exists but is inaccurate or implausible
P0397Cylinder 1 Pressure Sensor Circuit LowSignal voltage below threshold
P0398Cylinder 1 Pressure Sensor Circuit HighSignal voltage above threshold
P0399Cylinder 1 Pressure Sensor Circuit Intermittent/ErraticSignal drops out or becomes unstable
P039ACylinder 1 Pressure Too LowActual measured pressure below expectation
P039BCylinder 1 Pressure Too HighActual measured pressure above expectation
P039CCylinder 1 Pressure Variation Too LowInsufficient pressure variation
P039DCylinder 1 Pressure Variation Too HighExcessive pressure variation
P039ECylinder 1 Combustion PerformanceCombustion behavior outside expectation

This article should remain focused on the high electrical signal. P039B owns the actual high-pressure search intent.

Is P0398 a Camshaft-Position-Sensor Code?

No.

P0398 concerns the Cylinder 1 pressure-sensor circuit.

P0390 through P0394 cover Bank 2 Camshaft Position Sensor B:

P0395 begins the Cylinder 1 pressure-sensor code family.

Older or incomplete code lists may incorrectly identify later P039X codes as camshaft-sensor faults. Always verify the definition using current service information for the exact vehicle.

How Serious Is the P0398 Code?

P0398 is generally a moderate fault that should be diagnosed promptly.

The engine may continue operating by substituting a calculated pressure value or using information from the other cylinders. That does not mean the fault is harmless.

Loss of accurate Cylinder 1 pressure information may affect:

Stop driving if the engine develops:

Limited driving may be possible if the engine runs normally, but the code should not be ignored.

Common Symptoms of P0398

Possible symptoms include:

Some vehicles may substitute a calculated value and continue running normally. The Check Engine Light may be the only immediate indication.

If the engine cranks normally but does not start, use Engine Cranks but Won’t Start to separate the pressure-sensor fault from fuel, timing, security and mechanical problems.

Common Causes of P0398

Signal Circuit Shorted to Voltage

A damaged pressure-sensor signal wire may contact:

This can hold the signal at or above its maximum expected voltage.

Inspect areas where the harness passes near:

Open Signal Circuit

Many sensor-input circuits include an internal PCM pull-up voltage. If the signal wire becomes open or the sensor is disconnected, the PCM may see the signal rise toward the reference voltage.

An open circuit may result from:

This is why “circuit high” can be caused by an open wire rather than an actual external voltage source.

Electronics: making broken connections look fully energized since forever.

Open Sensor Low-Reference Circuit

The sensor low-reference circuit provides the controlled return path used by the PCM.

If that circuit opens or develops excessive resistance, the sensor signal may rise above its permitted range.

Possible causes include:

Sensor low reference is not always the same as chassis ground. Do not jumper the circuit to the engine without following the wiring diagram and manufacturer procedure.

Disconnected Pressure Sensor

A loose or disconnected pressure-sensing glow-plug connector may cause the signal voltage to remain high.

Check for:

Failed Cylinder 1 Pressure Sensor

The integrated electronics may fail and hold the signal near its upper electrical limit.

Possible internal failures include:

If the sensor is integrated into the glow plug, the complete pressure-sensing glow-plug assembly may require replacement.

Corroded or Spread Terminals

High terminal resistance or lost contact can make the circuit behave like an open connection.

Inspect for:

Do not force oversized meter probes into the connector. Turning a questionable terminal into a confirmed damaged terminal is not diagnostic progress.

Incorrect Pressure-Sensing Glow Plug

An incorrect replacement sensor may produce the wrong:

A conventional glow plug cannot replace a pressure-sensing version.

Verify the part number using the vehicle identification number and manufacturer catalog.

Improper Installation

Incorrect installation can damage the sensor or affect its output.

Possible installation errors include:

Follow the exact torque and installation procedure.

Shared Circuit Fault

The Cylinder 1 sensor may share reference or low-reference circuits with other sensors.

A wiring fault affecting the shared circuit can create several sensor codes simultaneously.

Check for related faults involving:

Multiple circuit-high codes are a useful clue that the problem extends beyond one sensor.

PCM Connector or Input Failure

PCM failure is possible but comparatively uncommon.

Before condemning the module, verify:

PCM replacement may require programming, coding and security initialization.

Diagnostic Tools You May Need

Diagnosing P0398 may require:

Do not apply conventional glow-plug testing methods to the pressure-sensor terminals unless the service procedure specifically permits it.

How to Diagnose the P0398 Code

1. Confirm the Definition

Verify that P0398 means Cylinder 1 Pressure Sensor Circuit High for the vehicle.

Do not rely solely on a generic scanner description. Manufacturer-specific service information should determine the circuit layout and test values.

2. Scan Every Control Module

Record:

Multiple high-voltage sensor codes may indicate a shared low-reference or wiring problem.

Browse the complete P-code library for related powertrain faults.

3. Check Battery and Charging Voltage

Verify:

Abnormally high charging voltage can affect sensor circuits and damage electronics.

If the battery warning light is illuminated, follow Why Is My Battery Light On? before assuming the pressure sensor is the only problem.

4. Review Freeze-Frame Data

Determine when P0398 set:

A code that sets immediately after key-on often indicates a hard electrical fault. A code that appears only when hot or during vibration may suggest a terminal, broken conductor or heat-sensitive sensor.

5. Identify Cylinder 1 Correctly

Cylinder numbering varies by engine design.

Use service information to identify:

Do not assume Cylinder 1 is always at the front of the engine.

6. Inspect the Sensor and Wiring

With the engine off and cool, inspect:

Pay particular attention to places where the signal wire could contact a reference-voltage or power circuit.

7. Compare Scan Data Between Cylinders

If individual cylinder-pressure data is available, compare Cylinder 1 with the other cylinders.

A circuit-high fault may appear as:

A fixed maximum value is more consistent with an electrical problem than genuine combustion pressure.

8. Check Reference Voltage

Measure the reference voltage according to the wiring diagram.

Possible results:

Do not assume every pressure sensor uses exactly 5 volts. Follow the manufacturer specification.

9. Measure the Signal Voltage

Back-probe the signal circuit using an approved method.

Compare the measurement with:

A signal fixed near the reference voltage may indicate:

10. Check the Low-Reference Circuit

Perform voltage-drop and continuity tests as specified.

Look for:

A low-reference wire can show continuity with a meter yet fail when the sensor is connected. Load testing may be required.

11. Test for a Short to Voltage

With the circuit powered down and isolated as specified, check whether the signal wire is shorted to:

Inspect harness contact points and previous repair areas carefully.

12. Check Signal-Circuit Continuity

Test continuity between the sensor connector and PCM.

An open or high-resistance signal circuit may cause the PCM input to rise because of its internal pull-up voltage.

Move the harness while monitoring resistance or voltage to reveal intermittent conductor damage.

13. Unplug the Sensor and Observe the Circuit

Depending on the manufacturer’s diagnostic procedure, monitor signal or reference voltage while disconnecting the Cylinder 1 sensor.

Possible results:

Interpret these results using the actual wiring diagram. Sensor circuits are not all designed identically.

14. Compare Oscilloscope Waveforms

Compare Cylinder 1 against a known-good cylinder during:

A healthy pressure waveform should rise and fall in synchronization with the engine cycle.

A P0398 waveform may show:

A genuine high-pressure condition should still produce a changing waveform. A circuit-high fault is more likely to remain electrically fixed or clipped.

15. Verify the Signal at the PCM

If the sensor output appears correct at its connector, measure the signal at the PCM.

16. Test or Replace the Sensor as Directed

Some pressure-sensing glow plugs require specialized testing and calibration.

When replacement is confirmed:

  1. Verify the exact part number.
  2. Allow the engine to cool.
  3. Clean contamination from around the sensor.
  4. Follow the specified removal procedure.
  5. Inspect the threads and sealing surface.
  6. Install the component using the correct torque.
  7. Reconnect the harness without twisting it.
  8. Perform required calibration or adaptation.
  9. Clear the codes.
  10. Verify the waveform and live data.

Common Diagnostic Results

Test resultLikely diagnostic direction
Signal fixed near reference voltageOpen signal, short to reference, open low reference or failed sensor
Signal high with sensor disconnectedHarness short, PCM terminal or PCM input issue
Signal returns to normal when sensor is disconnectedInternally failed sensor
Low reference shows excessive voltage dropOpen, resistance, damaged terminal or splice
Correct signal at sensor but high at PCMHarness or terminal fault
Multiple sensors report circuit highShared low-reference or supply-circuit problem
Cylinder 1 waveform stuck highHard electrical fault
Cylinder 1 waveform changes normallyCheck intermittent wiring, code history and PCM interpretation

Common Repairs for P0398

Repairs may include:

The correct repair must follow the test results. Replacing the sensor because the code contains the word “sensor” is still guessing, even when the guess arrives in a factory-branded box.

What Not to Replace First

P0398 does not automatically justify replacing:

These components may cause other faults, but P0398 directs diagnosis toward the Cylinder 1 pressure-sensor electrical circuit.

Can a Bad Injector Cause P0398?

An injector problem can create abnormal combustion pressure, but P0398 specifically indicates excessive sensor-circuit voltage.

A leaking or overfueling injector is more likely to cause:

Verify the electrical circuit before diagnosing the injector.

Can High Compression Cause P0398?

High actual cylinder pressure may produce a larger valid pressure waveform, but P0398 concerns voltage above the circuit’s permitted electrical threshold.

P039B is the more specific code for actual Cylinder 1 pressure that is too high.

If the sensor circuit and waveform are electrically correct but measured pressure is genuinely excessive, then investigate:

Can a Disconnected Sensor Cause P0398?

Yes.

On circuits using an internal PCM pull-up, disconnecting the sensor or opening the signal wire can allow the voltage to rise toward the reference-voltage level.

Always inspect connector engagement and terminal condition before replacing components.

Can a Bad Ground Cause P0398?

An open or high-resistance sensor low-reference circuit can cause the signal voltage to rise.

Do not confuse sensor low reference with the vehicle’s main engine ground. Follow the wiring diagram and measure voltage drop through the correct circuit.

Can a Pressure-Sensing Glow Plug Cause P0398?

Yes.

Its integrated pressure sensor may fail internally and hold the output voltage high. The heating element may continue operating normally.

Confirm the reference, signal and low-reference circuits before replacing the assembly.

Can Low Battery Voltage Cause P0398?

Low battery voltage is not the most direct cause of a circuit-high code, but system-voltage problems can affect module operation and create misleading faults.

Check battery and charging voltage before advanced diagnosis, especially when several unrelated codes are stored.

Can You Drive With P0398?

Limited driving may be possible if the engine starts normally, runs smoothly and produces no unusual smoke or noise.

Stop driving if the vehicle develops:

Accurate pressure data may be important for fuel control and emissions management, so the fault should be repaired promptly.

Estimated P0398 Repair Costs

RepairEstimated cost
Diagnostic scan and circuit testing$120–$300
Connector or terminal repair$100–$350
Local wiring repair$150–$500
Shared-circuit diagnosis$200–$600
Pressure-sensing glow-plug replacement$250–$800
Multiple pressure-sensing glow plugs$700–$2,000+
PCM connector repair$200–$700
PCM software update$150–$400
PCM replacement and programming$800–$2,500+
Mechanical cylinder-pressure diagnosis$200–$600

Costs vary substantially with accessibility and component design. A seized pressure-sensing glow plug can add considerable labor and risk.

Common P0398 Diagnostic Mistakes

Treating Circuit High as High Combustion Pressure

P0398 indicates high electrical voltage. It does not prove excessive pressure inside Cylinder 1.

Replacing the Sensor Before Checking the Connector

A disconnected or corroded terminal can create the same high-signal condition.

Ignoring the Low-Reference Circuit

An open sensor low reference can drive the signal high.

Looking Only for a Short to Voltage

Circuit-high codes can also result from open circuits.

Using a Conventional Glow Plug

A standard glow plug does not contain the required pressure sensor.

Assuming Cylinder 1 Location

Cylinder numbering varies by manufacturer and engine family.

Testing Only at the Sensor

Comparing measurements at the sensor and PCM can expose harness faults.

Replacing the PCM Too Soon

Test every wire, splice and terminal before condemning the module.

Using Oversized Test Probes

Spreading a terminal can create a new intermittent fault and make an already complicated diagnosis even more festive.

How to Confirm the Repair

After completing the repair:

  1. Reconnect every electrical connector.
  2. Secure the harness in its original routing.
  3. Verify battery and charging voltage.
  4. Clear stored and pending codes.
  5. Start the engine from cold.
  6. Monitor Cylinder 1 pressure-sensor voltage.
  7. Compare Cylinder 1 data with the other cylinders.
  8. Allow the engine to reach operating temperature.
  9. Perform a harness-movement test when appropriate.
  10. Road-test under the original freeze-frame conditions.
  11. Rescan for pending and stored codes.
  12. Confirm that P0398 does not return.
  13. Verify normal starting, idle and engine power.
  14. Confirm that readiness monitors complete.

A successful repair must restore a changing, plausible pressure signal. Clearing the code without fixing the circuit merely schedules the Check Engine Light’s return engagement.

Frequently Asked Questions

What does the P0398 code mean?

P0398 means the Cylinder 1 pressure-sensor circuit voltage is above the maximum level expected by the engine control module.

Is P0398 a camshaft-sensor code?

No. P0398 concerns the Cylinder 1 pressure sensor. P0390 through P0394 cover Bank 2 Camshaft Position Sensor B.

Does P0398 mean Cylinder 1 pressure is too high?

No. P0398 is an electrical circuit-high code. P039B is the dedicated code for actual Cylinder 1 pressure that is too high.

What is the difference between P0397 and P0398?

P0397 indicates pressure-sensor circuit voltage below threshold. P0398 indicates voltage above threshold.

What is the difference between P0398 and P0399?

P0398 describes a signal that remains electrically high. P0399 describes an intermittent or erratic signal.

Can an open wire cause P0398?

Yes. An open signal or low-reference circuit can allow the PCM input voltage to rise.

Can a disconnected pressure sensor cause P0398?

Yes. Depending on the circuit design, disconnecting the sensor may cause the signal to rise toward reference voltage.

Can a bad ground cause P0398?

An open or high-resistance sensor low-reference circuit can cause the signal voltage to remain high.

Can a pressure-sensing glow plug cause P0398?

Yes. The integrated pressure-sensor electronics may fail even when the glow-plug heating element still works.

Can a regular glow plug replace a pressure-sensing glow plug?

No. A conventional glow plug cannot provide the pressure signal required by the engine controller.

Can a bad injector cause P0398?

An injector can affect real combustion pressure, but P0398 specifically identifies high electrical signal voltage. Test the sensor circuit first.

Can I drive with P0398?

Limited driving may be possible if the engine runs normally. Stop driving if it develops severe vibration, heavy smoke, loud combustion noise or major power loss.

How do you fix P0398?

Test the signal, reference and low-reference circuits. Repair open or shorted wiring, restore terminal contact and replace the sensor only when testing confirms failure.

Final Thoughts

P0398 means the PCM sees excessive voltage in the Cylinder 1 pressure-sensor circuit. The most likely causes include an open signal wire, signal shorted to reference voltage, missing sensor low reference, disconnected connector or internally failed pressure-sensing glow plug.

Begin with a complete scan and freeze-frame review. Identify Cylinder 1 correctly, inspect the connector and compare the sensor voltage with a known-good cylinder. Test the signal and low-reference circuits at both the sensor and PCM before replacing anything.

Most importantly, keep the electrical and mechanical meanings of “high pressure” separate. P0398 says the circuit voltage is high. It does not authorize an injector replacement, cylinder-head removal or interpretive dance around the compression tester.

For additional diagnostic information, visit the DTC Doctor or browse the complete P-series OBD-II code library.

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