P0958

ASM Mode Circuit High

Powertrain Emission Controls Air Injection System 🟡 Moderate — Fix within a week ⚠️ Drive with Care
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What This Actually Means

In plain language — no jargon

Your engine's air-switching solenoid circuit is reading too high voltage, like a light switch stuck in the 'on' position. The ECU can't properly control the air injection system that reduces emissions.

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Symptoms You May Notice

3 known symptoms for this code
Check Engine Light illuminated
Rough idle or poor fuel economy
Failed emissions test
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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECM monitors the voltage signal from the Air Switching Solenoid (ASM) control circuit. When voltage remains abnormally high instead of cycling between on and off, the ECU cannot verify proper solenoid operation. This prevents the air injection pump from switching between upstream and downstream injection modes.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
ASM Circuit Voltage 0.5-4.5V (switching pattern) Sustained >4.8V or open circuit
Signal Response Time 50-200ms switching No voltage change detected
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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
ASM Solenoid wiring connector
Inspect and clean the connector pins for corrosion or loose contacts, then reseat firmly.
2
ASM Solenoid harness
Check for pinched, melted, or damaged wiring between solenoid and ECU, repair or replace as needed.
3
Air Switching Solenoid assembly
If wiring is clean and connections tight, replace the solenoid itself as it may be internally stuck or failed.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code P0958 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code P0958

What happens after you fix the fault

Once the fault is repaired, P0958 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.