What does it mean?
On vehicles with a stop/start system the control module remembers the crankshaft angle at which the engine came to rest during an automatic stop. Knowing that resting position lets it inject and fire into the correct cylinder immediately at the next automatic restart, which is what makes those restarts so quick and quiet. P034A is stored when the position the module memorised does not agree with the position the crankshaft sensor reports once the engine turns again, typically a difference of more than one reluctor tooth repeated over more than one restart in the same driving cycle. The module can no longer trust its rest position data, so it falls back to a conventional starter driven start and on many vehicles disables the stop/start function.
Symptoms
- Stop/start no longer works, or works only occasionally
- Automatic restarts become noticeably slower, rougher or noisier than they used to be
- MIL on with the code stored, sometimes together with a stop/start message in the display
- Occasional shudder or a brief hesitation as the engine restarts
- Rarely, a delayed conventional start after the engine has been switched off
- Engine performance while driving is otherwise normal
Likely causes
- Crankshaft position sensor with a weak, noisy or drifting output, particularly at the very low speeds reached during shutdown
- Damaged, cracked or contaminated reluctor ring, or metal debris caught between the teeth
- Loose sensor mounting or an incorrect air gap after previous repair work
- Wiring faults: chafed harness, damaged shielding, corroded connector or water in the plug
- Reluctor ring that has moved slightly on the crankshaft, or a sheared locating key
- Weak battery or a poor terminal connection causing low voltage during the stop and restart sequence
- Control module software that the manufacturer has since updated for this function
How to fix it
- Check the battery state of charge and its terminals first, since stop/start systems are very sensitive to voltage and many vehicles need the battery registered after replacement
- Read the freeze frame data and establish whether the code sets on every automatic restart or only sometimes
- View the crankshaft sensor signal on an oscilloscope through a full stop and restart cycle and look for dropouts and noise
- Inspect the sensor and its mounting for looseness, damage and correct seating, and check the air gap where a figure is specified
- Inspect the reluctor ring for cracks, missing or damaged teeth and for metallic debris on the sensor tip
- Test the sensor wiring for continuity, shield integrity, short to ground and voltage drop, wiggling the harness during the test
- Check for a manufacturer software update before replacing components, then clear the code and confirm over several stop/start cycles