P2340: Cylinder 5 Above Knock Threshold
The knock-sensor evaluation reports cylinder 5 above the knock threshold over a sustained window. The ECU retards ignition for that cylinder; persistent knock indicates poor fuel quality, carbon deposits or cooling issues.
What P2340 means
P2340 is a generic OBD-II powertrain code in the misfire & ignition group.
- Check engine light
- Usually on
- Emissions-related
- Yes — can fail inspection
- Limp mode possible
- Not typical
- Drive cycle to clear
- Required
Get it fixed
Post this code once and get quotes from local mechanics. Free to post.
Makes the repair and parts links below specific to your car. Kept in this browser only.
Common causes
- 01Low octane or contaminated fuelMost likely
- 02Carbon deposits raise compression locallyPossible
- 03Local cooling issue at cylinder 5Less likely
Can I drive with P2340?
Usually yes, for a short time.
The car will often drive normally, but get it diagnosed soon. Left alone it can raise fuel use and emissions and will fail an OBD inspection. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
Why we rate it moderate
- Turns on the check engine light
- Emissions-related: will usually fail an OBD inspection
- Needs a drive cycle to confirm a repair
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose P2340
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just P2340. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Check for related codes first
Look for P2341, P2342, P2343, P2339. Related codes often share a cause; a circuit or sensor code usually needs fixing before a performance code like this one.
- 3
Check live data
Watch the related sensor and actuator readings in live data (Knock sensor, Cylinder 5) and compare them with expected values in the service information for your vehicle.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Low octane or contaminated fuel; carbon deposits raise compression locally; local cooling issue at cylinder 5. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and complete a full drive cycle (cold start, idle, steady highway cruise, deceleration). The code is fixed when the related readiness monitor completes without it returning.
P2340 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Moderate
- Good DIY job?
- Yes, with a scan tool
Needs a scan tool and some experience with testing.
Parts and systems involved
- Knock sensor
- Cylinder 5
P2340 FAQ
What does P2340 mean?
P2340 means “Cylinder 5 Above Knock Threshold”. The knock-sensor evaluation reports cylinder 5 above the knock threshold over a sustained window. The ECU retards ignition for that cylinder; persistent knock indicates poor fuel quality, carbon deposits or cooling issues.
Is P2340 serious? Can I keep driving?
We rate P2340 as moderate severity. Usually yes, for a short time. The car will often drive normally, but get it diagnosed soon. Left alone it can raise fuel use and emissions and will fail an OBD inspection. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
What causes P2340?
The most common causes, from most to least likely: Low octane or contaminated fuel; Carbon deposits raise compression locally; Local cooling issue at cylinder 5. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix P2340?
Typical labor is about $65–$500 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix P2340 myself?
DIY difficulty is moderate. Many owners can handle it with a scan tool, a multimeter and basic hand tools. Always confirm the cause before buying parts.
Will P2340 clear itself?
Once the fault is fixed, the check engine light usually goes off by itself after the system passes its self-test over several drive cycles. Clearing the code with a scan tool resets readiness monitors, so an inspection station may turn you away until a full drive cycle is completed.