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Home / DTC / P1691 — MIL (Check Engine)

P1691 — MIL (Check Engine)

Detailed page for trouble code P1691.

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Code

P1691

PORSCHE P — Powertrain

MIL (Check Engine)

Brand: PORSCHE
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Emissions system fault (catalyst, oxygen sensors)
  • EVAP system leak or component failure
  • Ignition or fuel-related misfires
  • Faulty sensor (MAF, MAP, O2) or slow/erratic sensor response
  • Poor power/ground or corroded connector in MIL/ECU circuit
  • Instrument cluster or MIL lamp circuit fault

Symptoms

  • MIL / Check Engine Light illuminated
  • Possible reduced engine performance or limp-home mode (if related fault severe)
  • Failed or questionable emissions test
  • Related drivability issues (misfire, rough idle, hesitation) depending on underlying fault
  • Stored related diagnostic trouble codes and freeze-frame data

What to check

  • Connect an OBD-II scanner; read all stored, pending and historic DTCs and freeze-frame data
  • Record freeze-frame and monitor live data while engine at operating temperature
  • Check for multiple codes that indicate specific subsystem (O2, catalyst, EVAP, ignition)
  • Inspect fuel cap and EVAP system for obvious leaks or disconnected hoses
  • Visually inspect wiring, connectors and grounds for ECM, sensors and instrument cluster
  • Verify battery voltage and charging system are within spec

Signal parameters

  • Oxygen sensor voltages and switching frequency (V)
  • Short-term and long-term fuel trims (%)
  • Mass airflow (MAF) signal (g/s) or manifold pressure (MAP)
  • Catalyst efficiency or downstream O2 sensor response
  • Misfire counts per cylinder (counts/min)
  • EVAP purge/vent status and fuel tank pressure (kPa)

Diagnostic algorithm

  1. Read and record all DTCs, freeze-frame and live data. Note related codes that point to a subsystem. 2) Clear codes and attempt to reproduce the MIL event while monitoring live parameters. 3) If emissions-related codes present (O2, catalyst), perform sensor tests: monitor upstream and downstream O2 behavior, compare voltages and response times. 4) Check fuel trims, MAF/MAP and fuel pressure to rule out mixture issues that trigger MIL. 5) Inspect EVAP system if EVAP codes present: perform smoke test and verify fuel cap. 6) Inspect wiring, connectors and grounds for ECM and MIL lamp circuit; repair any damage or corrosion. 7) Verify proper battery/charging voltages and recheck communication on diagnostic bus. 8) If instrument cluster MIL circuit suspected, perform lamp/circuit tests per manufacturer procedure. 9) If no external cause found and manufacturer procedures point to ECM, consider reprogramming/ reflashing ECM software or replacing ECU per factory guidance. 10) After repairs, erase codes and verify the MIL stays off through sufficient drive cycles and that related parameters return to normal.
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Likely causes

  • Stored emissions-related DTC (e.g., O2 or catalyst efficiency) that required MIL illumination
  • Loose or faulty fuel cap causing EVAP fault
  • Failed oxygen sensor reporting out-of-range voltages
  • Degraded catalytic converter causing efficiency below threshold
  • Faulty ground or supply to ECM or instrument cluster
  • ECM internal fault or corrupted software

Fault status

⚠️ Status
MIL (Check Engine) illuminated — ECM has set a condition requiring driver notification. Additional stored DTCs and freeze-frame data are required to identify the underlying cause.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5 - 3.0 hours

Similar codes

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136

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