Symptom
Tachometer reading erratically or not at all
The rev counter jumps, drops to zero or does not move. Since the tachometer is usually driven from the crankshaft or ignition speed signal, this is a strong clue pointing at that signal rather than at the instrument.
Also described as: rev counter not working, tacho not working, rpm gauge erratic, tachometer dead, revs jumping, دور سنج کار نمیکند, عقربه دور موتور نامنظم, cuentavueltas no funciona, tacometro loco, aguja revoluciones loca, عداد الدورات لا يعمل, مؤشر RPM يتخبط, ساعة الدورات معطلة
Some causes mean you should stop driving
3 of the 18 codes that produce this symptom indicate a fault where continuing to drive risks damage or is unsafe. If the symptom is sudden or severe, treat it as one of those until a scan says otherwise.
What this usually means
Since the tachometer is usually driven from the crankshaft or ignition speed signal, this is a strong clue pointing at that signal rather than at the instrument. That is worth stating first, because the instinct is to suspect the gauge and the gauge is rarely the fault.
And the same signal is what the engine needs to run at all. The management will not fire injectors or coils without a crank position signal — so a rev counter that drops to zero while the engine is running is reporting something the engine is about to notice too. That makes an erratic tachometer an early warning of a stall rather than a cosmetic annoyance.
A tachometer that reads zero while the engine plainly runs is the classic crank sensor picture, and it is checkable in seconds: watch the live engine speed value on a scan tool. If that is also zero, the signal is genuinely missing; if it is correct, the fault is between the module and the cluster.
Erratic rather than absent points at a marginal signal — a damaged reluctor ring, a wide air gap from corrosion behind the sensor, or a chafed harness. These are frequently heat-dependent, so a car whose rev counter flickers after twenty minutes is describing a fault that will eventually stop the engine.
On an older vehicle the tachometer may be driven directly from the ignition coil, in which case the fault is genuinely in that circuit and not in the engine management at all — worth establishing before chasing sensors.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Reads zero while the engine is plainly running | The crank position signal missing — and the engine will notice too | Watch live engine speed on a scan tool. Zero there as well means the signal is genuinely absent. | Renew the sensor or repair the wiring, inspecting the reluctor before fitting. |
| Live engine speed correct, gauge wrong | The cluster or the message path to it rather than the sensor | The scan tool reading correctly is the discrimination. Check other gauges for a shared fault. | Diagnose the cluster. See the dashboard-gauges entry. |
| Flickers or drops after twenty minutes of running | A heat-dependent signal fault — the same one that eventually stalls the engine | Reproduce hot and log the live value. Treat this as a warning rather than a display fault. | Renew the sensor before it strands the vehicle. This pattern precedes a cut-out. |
| Erratic over bumps | A chafed harness or a connector that moves — mechanical rather than thermal | Wiggle-test the sensor harness while monitoring the live value. | Repair the harness and secure it clear of what it moves against. |
| Older vehicle with the tachometer driven from the ignition coil | That circuit rather than engine management | Establish how the vehicle drives its tachometer before chasing sensors. | Repair the coil feed or the gauge. The engine management reasoning does not apply. |
| Erratic alongside stalling or hard starting | The same signal fault, now affecting the engine as well as the gauge | Treat the two as one. Log engine speed through a stall event. | Renew the sensor. See the cuts-out-without-warning entry, which is the same fault further along. |
-
Reads zero while the engine is plainly running
- Likely cause
- The crank position signal missing — and the engine will notice too
- How to confirm it
- Watch live engine speed on a scan tool. Zero there as well means the signal is genuinely absent.
- What fixes it
- Renew the sensor or repair the wiring, inspecting the reluctor before fitting.
-
Live engine speed correct, gauge wrong
- Likely cause
- The cluster or the message path to it rather than the sensor
- How to confirm it
- The scan tool reading correctly is the discrimination. Check other gauges for a shared fault.
- What fixes it
- Diagnose the cluster. See the dashboard-gauges entry.
-
Flickers or drops after twenty minutes of running
- Likely cause
- A heat-dependent signal fault — the same one that eventually stalls the engine
- How to confirm it
- Reproduce hot and log the live value. Treat this as a warning rather than a display fault.
- What fixes it
- Renew the sensor before it strands the vehicle. This pattern precedes a cut-out.
-
Erratic over bumps
- Likely cause
- A chafed harness or a connector that moves — mechanical rather than thermal
- How to confirm it
- Wiggle-test the sensor harness while monitoring the live value.
- What fixes it
- Repair the harness and secure it clear of what it moves against.
-
Older vehicle with the tachometer driven from the ignition coil
- Likely cause
- That circuit rather than engine management
- How to confirm it
- Establish how the vehicle drives its tachometer before chasing sensors.
- What fixes it
- Repair the coil feed or the gauge. The engine management reasoning does not apply.
-
Erratic alongside stalling or hard starting
- Likely cause
- The same signal fault, now affecting the engine as well as the gauge
- How to confirm it
- Treat the two as one. Log engine speed through a stall event.
- What fixes it
- Renew the sensor. See the cuts-out-without-warning entry, which is the same fault further along.
Codes that produce this symptom (18)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0654 Engine RPM Output Circuit/Open Typical of this code Low
- P0737 Transmission Control Module Engine Speed Output Circuit Typical of this code Moderate
- P0738 Transmission Control Module Engine Speed Output Circuit Low Typical of this code Moderate
- P0739 Transmission Control Module Engine Speed Output Circuit High Typical of this code Moderate
- P2617 Crankshaft Position Signal Output Circuit Open Typical of this code Moderate
- P2618 Crankshaft Position Signal Output Circuit Low Typical of this code Moderate
- P2619 Crankshaft Position Signal Output Circuit High Typical of this code Moderate
- U0100 Lost Communication With Engine Control Module A Typical of this code Stop driving High
- U0155 Lost Communication With Instrument Panel Cluster (IPC) Control Module Typical of this code High
- P0335 Crankshaft Position Sensor A Circuit Stop driving Critical
- P0339 Crankshaft Position Sensor A Circuit Intermittent Stop driving Critical
- P0600 Serial Communication Link Moderate
- P0644 Driver Display Serial Communication Circuit Low
- P0727 Engine Speed Input Circuit No Signal High
- P0728 Engine Speed Input Circuit Intermittent High
- P2614 Camshaft A Position Signal Output Circuit/Open Bank 1 Moderate
- P2615 Camshaft A Position Signal Output Circuit Low Bank 1 Moderate
- P2616 Camshaft A Position Signal Output Circuit High Bank 1 Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others with this symptom
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about Tachometer reading erratically or not at all.