2002 Chevrolet Tahoe 5.3L P0300 Code & Clock Reset Issue
I have a 2002 Chevy Tahoe 5.3 LS. I got a “Service Engine Soon,” shut it off to scan codes, and noticed the clock reset to 1:00 like the battery was disconnected. I saw P0300, cleared it, and later at a red light it shook again. Now P0300 is pending and the clock reset to 1:00 again. AutoZone says the alternator tests good. Any suggestions?
You’ve got two symptoms tied together: random misfire (P0300) and the clock resetting like the battery was briefly disconnected. That points to a main power or ground interruption, not a typical engine or alternator fault. If the alternator tests good, I’d bet on a loose/corroded battery cable or a bad ground. If that’s not it, ignition switch is a runner-up, but less likely.
Most likely culprits
- Loose/corroded battery terminal or cable — Fits both the clock reset and random misfire from voltage drop.
- Bad main ground (engine to frame/body) — Can cause intermittent power loss, especially under load or vibration.
- Faulty ignition switch — Only if power loss happens with no battery/ground issues.
How to narrow it down
- Pop the hood, grab both battery terminals, and try to twist/wiggle them by hand.
- If you get movement, clean and retighten. Loose = likely culprit.
- Check for corrosion (white/green fuzz) on terminals and cable ends.
- Clean with a wire brush and baking soda/water if present.
- Follow the negative battery cable to where it bolts to the engine block and frame.
- Make sure both ends are tight and clean. Remove, wire-brush, and retighten if in doubt.
- With the engine running, have someone watch the dash while you gently tap or wiggle the battery cables and main ground.
- If the clock resets or dash flickers, you found your problem.
- If all cables are solid, try gently bumping the ignition key while running (don’t turn it off, just wiggle).
- If the dash resets, suspect the ignition switch.
- Scan for codes in the BCM (Body Control Module) and check for voltage drop events.
- If you see “low voltage” or “lost communication” codes, that’s more evidence of a power/ground issue.
Possible parts (if needed)
- Battery terminal/cable (if loose or corroded)
- Main ground cable (if corroded or loose)
- Ignition switch (only if power loss persists after battery/ground checks)
A few questions
- Have you noticed any other electrical weirdness (dome lights, radio, gauges flickering, etc.)?
- How old is the battery, and have the terminals ever been replaced?
- Did this start after you worked under the hood or after any jump-starts?
Get those basics checked—most of the time, it’s a battery cable or ground on these trucks. If you find a loose or corroded connection, fix that first before chasing anything deeper.