P0340 on a SBC 350: Camshaft Position Sensor and What Actually Causes It
TL;DR: P0340 is a generic OBD-II code meaning the computer is not seeing a usable camshaft position signal. It names a circuit, not a part. On a small block Chevy 350 the first job is establishing which 350 you actually have, because the cam signal comes from a different place depending on the induction and control system, and half the wrong-part purchases start with skipping that step. Once you know where the signal originates, the diagnosis is a short list: wiring and connector, power and ground, the reluctor or trigger the sensor reads, then the sensor itself. Replace the sensor first and you will often find yourself replacing it twice.
What P0340 actually tells you
P0340 is defined at the OBD-II generic level as a camshaft position sensor "A" circuit fault. Every manufacturer that follows the generic set uses it the same way: the powertrain control module expected a cam signal with a certain shape, timing and frequency, and did not get one it could use.
Notice what that does not say. It does not say the sensor failed. It says the circuit did not deliver. A circuit includes the sensor, the connector, the harness between the connector and the module, the power feed, the ground, the reference or signal-return line, and the physical thing the sensor is reading. Any one of those breaks the circuit, and all of them set the same code.
That is why P0340 has such a bad reputation for repeat repairs. It is an easy code to throw a part at, and the part is usually cheap enough that people do it without testing. Then the code comes back in a week because the real fault was a green pin in a connector or a chewed wire behind the intake.
The other half of the definition matters too: the module compares the cam signal against the crank signal. A cam signal that is present but wrong — arriving at the wrong point in the crank's rotation — can set P0340 or a related correlation code depending on the calibration. That opens the door to mechanical causes that have nothing to do with electronics, which we will get to.
First: figure out which 350 you have
"SBC 350" covers a lot of ground, and the cam signal source is not the same across all of it. Before you look up a sensor, establish what is in front of you.
A carbureted small block with a conventional points or HEI distributor and no engine computer has no camshaft position sensor and cannot set an OBD-II code at all. If you are reading P0340 on a vehicle with a 350 in it, one of the following is true:
- The engine is a factory fuel-injected 350 in an OBD-II-era vehicle, where the cam signal is generated at the distributor.
- It is a Gen II 350 with the front-mounted optical ignition unit, which produces both the ignition trigger and the cam-position information from one assembly.
- The engine has been swapped, re-powered or re-controlled — a crate engine, a marine or industrial package, or a standalone aftermarket EFI system — and the cam signal comes from whatever that harness and controller were designed around.
The practical consequence: confirm the sensor location and part number against the manufacturer catalogue for your exact engine and control system before ordering anything. Two 350s that look identical in the bay can use entirely different trigger arrangements. A parts lookup keyed only on "5.7L 350" is how the wrong sensor ends up on the bench.
If the engine is a swap, find out what the controller expects. Standalone systems vary widely in whether they need a cam signal at all, whether they need it for sequential injection only, and what they do when it goes missing. Some will run without it in a limp mode; some will not start. Your wiring diagram is the ECU manufacturer's documentation, not a factory manual.
Browse SBC 350 parts once you have confirmed the configuration, and cross-check the engine sensors listing against your control system rather than the engine displacement alone.
Symptoms that point at a cam signal problem
P0340 does not always come with a dramatic driveability complaint, which throws people off. What you see depends on how the controller handles a lost cam signal.
| Symptom | What it suggests | Where to look first |
|---|---|---|
| Extended crank, starts eventually | Module waiting to sync, then falling back to a default strategy | Signal present but intermittent or weak |
| No start, cranks normally, no spark or no injector pulse | Complete loss of sync on a system that requires cam input | Power, ground, open circuit, sensor |
| Runs, sets code, mild loss of power | Controller running in a batch-fire or wasted-spark fallback | Intermittent signal, connector, harness chafe |
| Stalls when hot, restarts when cool | Heat-sensitive electronics or a marginal connection | Sensor, connector terminal tension |
| Code returns after a sensor replacement | The sensor was never the fault | Wiring, reluctor, mechanical timing |
The last row is the important one. A repeat P0340 after a new sensor is a strong signal to stop buying parts and start testing the circuit.
The diagnostic order that actually saves money
Work outside-in. Every step here is cheaper than the one after it.
1. Read all the codes and the freeze frame. P0340 by itself reads differently from P0340 alongside a crank sensor code, a correlation code, or a systemwide loss of reference voltage. Multiple sensors dropping out together almost always means a shared power or ground, not multiple failed sensors.
2. Inspect the connector. Unplug it and look at the terminals under good light. Green or white corrosion, a terminal pushed back out of the housing, spread female terminals with no grip, oil or coolant wicked into the connector body — these are common and they are cheap. Check that the locking tab is actually latched.
3. Wiggle-test the harness with the scan tool live. Watch the cam signal parameter, or the RPM signal if the tool shows it, while you move the harness along its run. A signal that drops when you flex a section has told you exactly where the break is. Pay attention to anywhere the harness crosses a sharp edge, passes near an exhaust component, or has been disturbed by previous work.
4. Verify power and ground at the sensor connector. Most cam sensors need a supply, a ground and a signal return. If any of the three is missing at the connector, the sensor is not the problem. Test with the connector loaded rather than open-circuit where the tool allows it — a corroded wire can show good voltage with no load and collapse under current.
5. Check the signal itself. A scope on the signal wire tells you far more than a meter. You are looking for a clean, consistent pulse train with full amplitude, no dropouts and no noise riding on it. A signal that is present but ragged points at the reluctor, the air gap or a shielding problem.
6. Look at what the sensor reads. A cam sensor is only as good as the trigger it sees. Debris on a reluctor, a damaged tone ring, a distributor with worn bushings letting the shaft wander, or excessive endplay in the drive can all corrupt an otherwise perfect signal. On a distributor-driven setup this is worth a hard look, because a worn distributor gear or bushing produces exactly the wandering signal that sets this code.
7. Consider mechanical timing. If the cam signal is clean but arrives at the wrong point relative to the crank, look at the timing set. A stretched chain, a worn set with excessive slack, or a set installed one tooth off will move cam timing relative to crank timing. Verify cam timing against the manufacturer's procedure and specification before condemning electronics. If the set is due, timing chains, gears and tensioners is the right place to start, and the broader small block Chevy engine parts range covers the related hardware you may want to replace while you are in there.
8. Then, and only then, replace the sensor. By this point you have a reason to.
What this typically costs in shop time
A competent diagnosis of P0340 is usually well under an hour of shop labour when the fault is in the connector or the harness. The sensor replacement itself ranges from a fifteen-minute job on an accessible sensor to a couple of hours if intake or accessory removal is required to reach it.
Where the bill grows is mechanical. If diagnosis leads to a worn distributor drive or a timing set, you are into front-cover work, and that is a multi-hour job on any small block. It is also the point where it makes sense to replace related wear items in one go rather than opening the front of the engine twice.
Parts strategy: what to buy and what not to
Buy the sensor once. A position sensor lives in heat and vibration and produces a signal the entire engine depends on for sync, and a marginal one causes intermittent faults that are miserable to chase. Confirm the current part number against the manufacturer catalogue for your specific application before ordering, and if the engine has a non-factory control system, confirm against the ECU manufacturer's specification instead.
Do not replace the crank sensor at the same time "just in case" unless testing pointed at it. Two changed variables means you learn nothing when the symptom moves.
Do replace the connector or pigtail if the terminals are corroded or loose. A new sensor plugged into a bad connector is a new sensor you will remove again.
If the diagnosis lands on the distributor as a mechanical problem, treat it as an ignition job rather than a sensor job — the ignition components range is where the cap, rotor and related hardware live, and doing it once with good parts is the point.
Frequently Asked Questions
Can I drive with a P0340 code? Sometimes, but you should not plan on it. If the engine runs, the controller has fallen back to a default strategy, and that strategy is a compromise. You may lose sequential injection accuracy, fuel economy and power, and the engine can stall without warning if the signal drops entirely. Diagnose it promptly.
Does P0340 always mean the camshaft position sensor is bad? No. It means the circuit did not deliver a usable signal. Wiring, connectors, power, ground, the trigger the sensor reads and mechanical cam timing can all set the same code. The sensor is one candidate among several, and it is not the most common one.
Why does my 350 have no camshaft position sensor at all? Because a carbureted small block with a conventional distributor does not need one. Cam position sensing arrived with engine management that fires injectors sequentially and controls spark electronically. If your 350 is carbureted and has no engine computer, it cannot set an OBD-II code.
I replaced the sensor and the code came back. What now? Stop replacing parts and test the circuit. Check power, ground and signal at the connector, wiggle-test the harness with live data, and scope the signal. A repeat code after a new sensor almost always means the fault was upstream in the wiring or downstream in what the sensor reads.
Could a stretched timing chain cause P0340? It can contribute. If the chain has enough slack to let cam timing drift relative to crank timing, the signal arrives at the wrong moment and the module may flag it. Verify cam timing against the manufacturer's procedure before assuming the electronics are at fault, particularly on a high-mileage engine.
Do I need a scan tool to diagnose this properly? You need something that reads live data, and a scope makes the job far easier. A code reader that only pulls and clears codes will tell you P0340 exists and nothing else. The useful information is in the live signal and how it behaves when you disturb the harness.
Sources
Written against the generic OBD-II code definition for P0340 (camshaft position sensor "A" circuit), general GM small block Chevy service practice, and standard electrical diagnostic method for position-sensor circuits. Application-specific sensor locations, part numbers, air gaps, resistance values and torque specifications were deliberately not stated here because they vary by model year, induction system and control system — confirm those against the factory service information for your exact engine, or the ECU manufacturer's documentation on a swapped or standalone setup.
Questions about which sensor fits your build? Email sales@coretransmissionparts.com. Free shipping on orders over $70.