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2020 Sierra 1500 Intermittent No-Start - Cranks but won't catch

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Mike T. · rep 0 · May 25, 2:35 AM · 1 replies · solved · Question

Vehicle: 2020 GMC Sierra 1500 AT4

Hey everyone, I'm having a really frustrating issue with my 2020 GMC Sierra 1500 AT4 (5.3L V8). About once a week, it just won't start. It cranks perfectly fine, but it just won't catch and fire up. It sounds like it's trying, but nothing.

Then, after sitting for anywhere from 10 minutes to an hour, it'll start right up like nothing ever happened. There's no Check Engine Light, and I've scanned it for codes, but nothing comes up, not even pending ones. The battery is relatively new and tests good, and I've checked the terminals to make sure they're clean and tight.

This is completely intermittent and unpredictable. Any ideas on what could be causing this? It's left me stranded a couple of times now.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:35 AM

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ClearTheCode admin answer:

An intermittent no-start condition where the engine cranks but doesn't fire up can be particularly challenging to diagnose, especially without any diagnostic trouble codes (DTCs). This usually points to a problem with one of the fundamental elements required for combustion: fuel, spark, or air, or possibly a security system issue.

Here's a systematic approach to narrow down the cause:

  1. Check for Fuel Pressure: When the no-start condition occurs, try to listen for the fuel pump priming sound (a brief whirring from the rear of the truck) when you turn the key to the ON position (before cranking). If you don't hear it, the fuel pump or its relay might be failing. A fuel pressure gauge can confirm if adequate pressure is reaching the fuel rail during the no-start event.
  2. Verify Spark: While cranking during a no-start, you need to confirm if spark is present at the spark plugs. This can be done with a spark tester. No spark could indicate issues with the ignition coils, crankshaft position sensor, camshaft position sensor, or even the engine control module (ECM).
  3. Inspect Crankshaft Position Sensor (CKP) and Camshaft Position Sensor (CMP): These sensors are crucial for telling the ECM when to fire the spark plugs and inject fuel. An intermittently failing sensor might not always set a code immediately but can cause no-start issues. Look for any signs of damage or loose connections.
  4. Security System (PassKey/Immobilizer): Modern GMC vehicles have security systems that can prevent the engine from starting if they don't recognize the key or detect tampering. If the security light flashes or stays on during a no-start, this could be the culprit. Try a different key if you have one.
  5. Relay Issues: The fuel pump relay, ignition relay, or even the ECM relay can fail intermittently. Swapping relays with a known good, identical relay (e.g., horn relay) can sometimes help diagnose this.

When to See a Shop:

Because this issue is intermittent and doesn't throw codes, it can be difficult to diagnose without specialized tools. If you're not comfortable testing fuel pressure or spark, or if the issue persists after basic checks, it's best to take your Sierra to a qualified technician. They have advanced diagnostic equipment that can monitor live data during a no-start event, check for communication issues, and perform more in-depth electrical testing to pinpoint the exact failure point.

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