P0370 — Timing Reference High Resolution Signal A

What Does P0370 Mean?

P0370 is a generic powertrain DTC indicating: Timing Reference High Resolution Signal A.

Engine misfires are detected by monitoring crankshaft rotational speed variations. Each cylinder's power stroke produces a characteristic acceleration pattern. When a cylinder fails to fire, the crankshaft decelerates during that cylinder's stroke. The ECM identifies which cylinder misfired and how often. P0313-P0399 codes cover extended misfire monitoring including lean misfire detection, knock-related ignition faults, and secondary ignition monitoring. A flashing check engine light during active misfires is an emergency — raw unburned fuel enters the catalytic converter, potentially destroying it.

Symptoms of P0370

⚠️ Check engine light on (may flash during active misfire)
⚠️ Engine misfiring — rough idle, stumbling, shaking
⚠️ Loss of power during acceleration
⚠️ Increased emissions and fuel consumption
⚠️ Catalytic converter damage if ignored
⚠️ Possible engine vibration at idle

Common Causes

🔍 Worn or fouled spark plugs
🔍 Faulty ignition coil
🔍 Vacuum leak causing lean misfire
🔍 Fuel injector issue (clogged, leaking, or electrical)
🔍 Low compression (worn rings, burned valve, head gasket)
🔍 Incorrect ignition timing

Diagnostic Tips

Check misfire counters in Mode 6 data — identifies which cylinder(s) are misfiring most
Swap ignition coil to another cylinder — if misfire follows, coil is bad
Check spark plugs — worn electrode, fouled with oil or carbon, cracked insulator all cause misfires
Perform compression test on affected cylinder(s) — low compression means mechanical issue
Check for vacuum leaks with a smoke machine — lean misfires are common

Common Mistakes to Avoid

❌ Replacing coils without checking spark plugs — a worn plug will kill a new coil
❌ Ignoring a flashing check engine light — this means severe misfire that's damaging the catalytic converter NOW
❌ Replacing ignition parts when the misfire is caused by a vacuum leak or low compression
❌ Not checking fuel trims — positive fuel trim + misfire often means vacuum leak, not ignition

Frequently Asked Questions

What does a flashing check engine light mean?

A flashing CEL means severe, active misfire that risks catalytic converter damage. Pull over safely and reduce engine load. Do not rev the engine. Drive gently to a repair facility or have it towed. This is the only CEL condition that warrants immediate action.

Can misfires damage my engine?

Misfires themselves don't usually damage the engine, but they can destroy the catalytic converter ($1000-$3000). Severe misfires also dilute engine oil with raw fuel, reducing lubrication. Extended misfires should always be repaired promptly.

Why does my car misfire only when cold?

Cold misfires are often caused by slightly worn spark plugs (gap too wide for cold, dense air-fuel mixture), marginal ignition coils that work fine once warm, or small vacuum leaks that are more noticeable at cold idle RPM.

What To Do Next

Possible Fixes

🔧 Replace spark plugs
🔧 Replace ignition coil(s)
🔧 Repair vacuum leak
🔧 Clean or replace fuel injector
🔧 Engine mechanical repair (valves, rings, gasket)

1
Find your vehicle above
Click your make and model for real owner reports and common causes specific to your vehicle.
2
Check for recalls
Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.
3
Get a professional diagnosis
A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.

P0370: Timing Reference High Resolution Signal A