Comprehensive Diagnostic Guide for OBD-II Code P0548

What This Code Means

• Primary definition: Exhaust Gas Temperature (EGT) Sensor Circuit High Input (Bank 1 Sensor 1). This designation is commonly listed under OBD-II Powertrain Codes.

• Open-source definition note: An alternate open-source listing describes (Low signal from the exhaust gas temperature sensor, bank 1), highlighting a discrepancy between sources. In practice, P0548 is widely treated as an EGT sensor circuit fault related to the sensor input level (high) for Bank 1 Sensor 1. See sources section for attribution.

• Location context: Bank 1 Sensor 1 indicates the EGT sensor located in Bank 1, Sensor 1 position (typically the sensor upstream of the on Bank 1). Location specifics vary by engine design; consult the vehicle service manual for exact placement.

• Source notes:

  • Wikipedia's OBD-II pages describe DTCs and powertrain codes at a general level and discuss how the OBD-II system monitors sensor circuits and flags P-codes when measurements deviate from the specification.
  • The Open Source (MIT) definition provided lists a different symptom terminology (low signal) but is not the consensus standard; the discrepancy is called out in this guide.

Symptoms

• Check Engine Light (MIL) is illuminated with code P0548 stored.
• Engine may run roughly or feel "off" during idle or acceleration, especially under load.
• Degraded or inconsistent acceleration response; possible hiccups or stumble at certain RPM ranges.
• Reduced fuel economy or altered exhaust behavior (possible due to ECU modifying fueling/ignition in response to sensor inputs).
• In some cases, no noticeable drivability issue beyond the MIL; the vehicle may still pass basic driving tasks.

General diagnostic philosophy (as a safety-focused guide)

• Treat P0548 as an issue in the EGT sensor circuit that can affect exhaust temperature readings and, consequently, ECU control strategies.
• Do not assume the sensor is at fault without verifying wiring, connections, and the ECU input channel first.
• Consider alternate causes (wiring, connectors, ECU input) before replacing the sensor, then re-check to confirm the fault is resolved.

Probable Causes

• Faulty EGT sensor (Bank 1 Sensor 1) or sensor that reads out-of-range consistently

  • Estimated probability: 35-50%

• Wiring harness damage or poor/shorted connections (including pins, harness abrasion, or connector corrosion)

  • Estimated probability: 20-35%

• PCM/ECU input fault, or software/calibration issue

  • Estimated probability: 5-10%

• Heater circuit fault (if the sensor uses a heater circuit; depends on sensor type)

  • Estimated probability: 5-15%

• Upstream exhaust condition or sensor installation issues

  • Estimated probability: 5-10%

• Other nearby sensor interactions (less common)

Safety and cautions

• Ensure the engine is off and cooled before touching exhaust components or wiring near hot areas.
• When testing electrical circuits, use proper PPE and avoid shorting circuits. Disconnect battery if performing invasive electrical tests as needed.
• If working near the exhaust system, ensure the vehicle is securely supported and that there is no risk of movement or burns.

Step-by-Step Diagnosis

1) Confirm and contextualize the code

• Use an OBD-II scanner to confirm P0548 is present and note any freeze-frame data.
• Check for other codes (especially P0545, P0546, P0547, P0549, or sensor heater codes) that could indicate related circuit issues.
• Review live data for Bank 1 Sensor 1 (EGT sensor signal) and any related sensor data (engine load, RPM, temperature, etc.).

2) Visual inspection and basic wiring checks

• Inspect the Bank 1 Sensor 1 wiring harness and connector for signs of damage, crushed insulation, corrosion, or oil/contaminant exposure.
• Disconnect and reseat the sensor connector; look for bent pins or loose connections.
• Check for obvious exhaust leaks upstream that could affect sensor readings (presence of soot, black corrosion, or soot at the upstream flange).

3) Baseline electrical checks (sensor circuit)

• With ignition ON but engine OFF, verify the sensor circuit wiring continuity to the ECU (signal wire, reference/ground as applicable). Compare resistance/voltage to service spec if available.
• For 2-wire vs 3-wire sensor configurations:
  • If 3-wire (signal, ground, heater): verify supply voltage to the heater circuit and resistance of the heater. A short or open in the heater can affect readings or safety.
  • If 2-wire (signal and ground): inspect the signal line integrity and the ground path; check for circuit contamination (low or high signal could be caused by ground issues).

4) Check sensor electrical performance with live data

• Monitor the EGT signal becoming high or unstable as the engine warms up to operating temperature. A consistently high signal (or a signal that rapidly hunts or sits at an elevated level) supports a sensor or circuit fault.
• Compare Bank 1 Sensor 1 reading to acceptable reference values from service information for your vehicle, if available. If the sensor reading is out of range consistently across driving cycles, suspect sensor or wiring.

5) Heater circuit and continuity testing (if applicable)

• If your vehicle uses a heated EGT sensor, test heater supply voltage and heater resistance against specification.
• A stuck-on heater or open heater circuit can cause abnormal readings or ECU fault triggers; repair as needed.

6) Exhaust condition check

• Inspect the exhaust manifold and upstream piping for leaks or damage which could affect sensor readings (intake of ambient air, altered temperatures).
• Verify sensor installation torque and thread integrity. A loose or misaligned sensor can produce incorrect readings.

7) Cross-check with bank 2 or other sensors (if applicable)

• If the vehicle has a Bank 2 EGT sensor or multiple banks, compare readings to see if Bank 2 shows similar symptoms; this can help determine if the issue is sensor-specific or a system-wide problem.

8) ECU and module considerations

• If all sensor and wiring tests pass but P0548 persists, consider ECU side fault (input circuit). In some cases, reflashing or programming updates from the OEM may address edge-case misinterpretations.
• If available, perform an advanced diagnostic with OEM diagnostic software to check for ECU fault codes related to the input channel.

9) Repair actions (based on test outcomes)

• Faulty Bank 1 Sensor 1 EGT sensor: Replace with an OEM-equivalent sensor. Follow the factory torque specs and installation procedures. Re-check after replacement with a drive cycle.
• Damaged wiring or connector: Repair or replace wiring harness segment; clean or replace connector; reseat; apply dielectric grease if recommended by OEM.
• Heater circuit fault: Repair or replace heater element or related wiring; retest (if applicable).
• ECU/input fault: Depending on scope, pursue ECU service or software update as per OEM guidance; re-test after any action.

8-step practical test plan (condensed)

• Step A: Read codes and freeze-frame data; note any related P0 codes.
• Step B: Visually inspect Bank 1 Sensor 1 wiring/connector; reseat connectors; look for corrosion or damage.
• Step C: Check signal and ground continuity; inspect for shorts-to- voltage or open circuits.
• Step D: If sensor is 3-wire: test heater circuit (voltage and resistance); if 2-wire: test signal path integrity.
• Step E: Observe live EGT sensor data while warming engine; check for abnormally high or unstable readings.
• Step F: Check for exhaust leaks upstream or improper sensor installation.
• Step G: If needed, compare readings with Bank 2 (if present) to determine if the fault is sensor-specific.
• Step H: Replace the faulty sensor or repair wiring as indicated; clear codes and perform a road test.
• Step I: Recheck for DTCs after a complete drive cycle to confirm repair success.

What to measure and expected observations (guidance)

• EGT sensor signal on Bank 1 Sensor 1: Abnormally high or erratic readings, especially once the engine reaches operating temperature, supports sensor or circuit fault.
• Sensor resistance (when vehicle is cool and sensor accessible): Deviant resistance values from spec indicate a faulty sensor or heater element (if applicable).
• Sensor heater circuit (if present): Confirm proper supply voltage and heater resistance; a fault here can cause abnormal sensor behavior or continued fault signaling.

Related codes and considerations

• Other EGT-related codes in the P054x family (e.g., P0545, P0546, P0547, P0549) may indicate similar but bank-specific or sensor-specific circuit faults. If multiple related codes appear, inspect both banks or the common ECU input.
• , the coding and interpretation of EGT sensor circuits are part of the broader OBD-II Powertrain Codes and Diagnostic Trouble Codes framework.

Repair Options

• Use OEM or manufacturer-specified replacement parts for EGT sensors and connectors.
• Follow torque specifications for sensor installation to avoid damage to threads or the sensor itself.
• When repairing wiring, use OEM-compatible wiring gauges and connectors; avoid reusing damaged connectors.
• After repair, perform a complete drive cycle to re-run readiness monitors and confirm the DTC does not return.

Documentation and testing records

• Record diagnostic steps taken, test results, and replacement parts (part numbers, lot numbers if relevant).
• Document the drive cycle results and readiness monitor status to verify repair integrity.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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