Every diagnostic trouble code guide on FullSizeChevy, grouped by system. Pull the code first with a scanner, then work the guide — most of these have a cheap, common cause that gets skipped because somebody guessed at parts instead.
Unless a guide says otherwise, these codes read the same across the GM full-size range. A P0420 means the same thing on a 1999 Silverado as it does on a 2021. Where a fix is genuinely platform-specific, it is called out in its own section further down.
Misfire codes
P0300 is a random or multiple-cylinder misfire. P0301 through P0308 name the exact cylinder. If you have a cylinder-specific code, start there — it narrows the job enormously.
| Code | What it means | Guide |
|---|---|---|
| P0300 | Random / multiple cylinder misfire | P0300 diagnosis and fix |
| P0301 | Cylinder 1 misfire | P0301 diagnosis and fix |
| P0302 | Cylinder 2 misfire | P0302 diagnosis and fix |
| P0303 | Cylinder 3 misfire | P0303 diagnosis and fix |
| P0304 | Cylinder 4 misfire | P0304 diagnosis and fix |
| P0305 | Cylinder 5 misfire | P0305 diagnosis and fix |
| P0306 | Cylinder 6 misfire | P0306 diagnosis and fix |
| P0307 | Cylinder 7 misfire | P0307 diagnosis and fix |
| P0308 | Cylinder 8 misfire | P0308 diagnosis and fix |
Fuel trim codes
| Code | What it means | Guide |
|---|---|---|
| P0172 | System too rich, Bank 1 | P0172 diagnosis and fix |
| P0175 | System too rich, Bank 2 | P0175 diagnosis and fix |
Catalytic converter codes
A catalyst code is not automatically a converter. Both guides work through the upstream causes before the expensive part.
| Code | What it means | Guide |
|---|---|---|
| P0420 | Catalyst efficiency below threshold, Bank 1 | P0420 diagnosis and fix |
| P0430 | Catalyst efficiency below threshold, Bank 2 | P0430 diagnosis and fix |
EVAP and emissions codes
EVAP work starts at the gas cap and moves outward. Do it in order and most of these never cost more than a solenoid.
| Code | What it means | Guide |
|---|---|---|
| P0442 | Small EVAP leak | P0442 diagnosis and fix |
| P0449 | EVAP vent solenoid circuit | P0449 diagnosis and fix |
| P0455 | Large EVAP leak | P0455 diagnosis and fix |
| P0496 | EVAP high purge flow | P0496 diagnosis and fix |
Camshaft timing codes
| Code | What it means | Guide |
|---|---|---|
| P0011 | Intake camshaft position timing — over advanced | P0011 diagnosis and fix |
| P0014 | Exhaust camshaft timing — over advanced | P0014 diagnosis and fix |
Cooling, sensor and idle codes
| Code | What it means | Guide |
|---|---|---|
| P0118 | Engine coolant temperature sensor, high input | P0118 diagnosis and fix |
| P0128 | Coolant temp below thermostat regulating temperature — usually a stuck-open thermostat | P0128 diagnosis and fix |
| P0507 | Idle air control — idle speed higher than expected | P0507 diagnosis and fix |
Platform-specific code guides
These cover the same codes but on one platform, where the fix or the failure point is specific enough to be worth its own guide.
- GMT800 EVAP codes P0442, P0446 & P0455 — gas cap to vent solenoid, in the order worth checking (1999–2006)
- GMT800 knock sensor codes P0327 & P0332 — corroded sensors in the valley under the intake (5.3, 6.0 Vortec)
- 5.3L V8 misfire (P0301): finding a bad ignition coil — a real diagnosis worked through in order on a 2014 Sierra 1500 L83
Warning lights and symptoms that are not a code
Not everything that lights the dash sets a P-code you can look up. These are the common ones.
- Service StabiliTrak — every cause, the DTCs behind it, and repair costs
- 5.3 ticking noise at idle — five very different causes, from a $5 exhaust leak upward
- 4L60E slipping in 3rd gear — five common causes, from a $30 solenoid to a rebuild
- AFM / DOD lifter failure — is your 5.3 ticking, or is it the lifters?
- Service 4WD light, 4WD not engaging — encoder motor and TCCM diagnosis
Still stuck?
Post the code, your year and engine, and what the truck is actually doing in the FullSizeChevy forum. Somebody there has chased the same code on the same platform. Freeze-frame data and fuel trim numbers help more than anything else you can post.