P1321–P1323 Knock Sensor 3 (G198) Circuit: Low Input
The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.
Can you drive it? This class of fault can cause real mechanical damage if you keep driving on it. A persistent misfire also destroys the catalytic converter, which is the expensive part.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P1321 | Knock Sensor 3 (G198) Circuit: Low Input | Do not keep driving |
P1323 | Knock Sensor 4 (G199) Circuit: Low Input | Do not keep driving |
The definition
P1321: Knock Sensor 3 (G198) Circuit: Low Input.
P1321 is a manufacturer-specific powertrain code (the engine or gearbox). Its meaning is set by the carmaker rather than by the SAE standard, so a generic OBD-II reader will often show it as an unknown code or misread it.
What to check first
Cheapest and fastest checks first:
- Unplug the sensor. If the reading jumps to full scale, the wiring is fine and the sensor has failed low.
- Check the sensor's 5V supply and its earth before condemning the sensor itself.
What actually causes this on a VAG
Knock sensors are microphones bolted to the block, listening for pinking, and the ECU trims ignition timing on what it hears. Knock sensor 3, G198, only exists on engines fitted with three or more of them - in the VAG range that means the big multi-cylinder engines such as the V8s, V10s and W12s, which typically carry four sensors, two per bank. The majority of four-cylinder and V6 VAG engines have only two knock sensors and cannot set this code at all, so if a scan tool reports it on a four-pot, be suspicious of the tool's code list before you go hunting for a sensor that is not there. "Low input" means the signal is sitting below anything plausible, which points at a short to earth, a collapsed screen in the cable, or a dead sensor. The engine will still run safely, just with timing pulled back on that bank, so it feels a little softer and drinks a bit more.
Most likely causes, in order
- Wiring or connector on the awkward bank. On the V engines the sensors live deep in the vee under the intake manifold, where their sub-loom rubs on brackets and studs and their connectors collect oil and water because they sit where everything drains. Check where the loom leaves the sensor and every point it is clipped down or crosses a metal edge. This is the cheapest thing to find and by a distance the most likely.
- Damaged screening or a bodged earlier repair. Knock sensor cable is screened; if the screen is broken or a previous repair used plain wire, the circuit picks up interference and reads implausibly. Look for insulation tape, heatshrink or scotchloks anywhere in the run - any of them is a strong clue, especially on a car whose engine has been out.
- Lead pinched at the sensor's mounting bolt. A short to earth has to touch metal. If this appeared after work in the vee, check the sensor lead has not been trapped under the sensor body or dragged tight over the bolt head as it was torqued down.
- Sensor failed internally. The piezo element can break down and short to the sensor body, which is earthed through the block. Prove it before you buy: unplug the sensor and check for continuity between each pin and the sensor body - a good one shows none. If the sensor is clean but the loom side of the plug shows continuity to earth, the fault is in the wiring and a new sensor changes nothing.
Worth knowing: Before you commit to a part, check your own engine's wiring diagram for which physical sensor is number three - the numbering is not laid out the same way on every engine, and on the V engines the sensors are identical parts, so it is easy to end up working on the wrong one. That matters here because the labour is the whole cost: reaching a bank 2 knock sensor usually means the intake manifold comes off. Do the continuity test at the plug first, from up top if you can reach it, so you know whether you are buying a sensor or repairing a loom before anything is dismantled.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
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Common questions
What does P1321 mean?
P1321 means Knock Sensor 3 (G198) Circuit: Low Input. The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.
Can I drive with P1321?
This class of fault can cause real mechanical damage if you keep driving on it. A persistent misfire also destroys the catalytic converter, which is the expensive part.