P0370 – Timing Reference High Resolution Signal “A”

P0370 is a trouble code you’ll most often see on diesel engines, especially older GM, Ford, and some European models that use an optical or crankshaft position sensor to control injection timing. When this code sets, your engine computer is telling you it’s not happy with the timing reference signal it uses to know when to fire the injectors. You might notice hard starting, rough running, or a lack of power. If you catch it early, repairs are usually straightforward. Ignoring it can lead to poor performance and potential engine damage.

What Does P0370 Mean?

P0370 stands for “Timing Reference High Resolution Signal A – Malfunction.” In plain language, your ECU is seeing an incorrect or unstable signal from a critical timing sensor. On many diesels, this is an optical sensor inside the injection pump or a crankshaft position sensor that provides a high-resolution signal so the ECU knows exactly when to inject fuel.

When that signal is missing, noisy, or out of range, the ECU can’t accurately control injection timing. That leads to hard starts, misfires, smoke, and sometimes a no-start condition. The code doesn’t always mean the sensor is bad; wiring, connectors, or even internal pump issues can also trigger P0370.

Quick Reference

• Code: P0370 – Timing Reference High Resolution Signal A Malfunction
• System: Engine timing / fuel injection control
• Typical engines: Primarily diesel, some older gasoline systems
• Severity: Moderate to severe, can lead to no-start
• Common culprit: Optical sensor or crankshaft position sensor and wiring

Real-World Example / Field Notes

In the shop, I see P0370 most often on older GM 6.5L diesels and some Ford Power Stroke and VW/Audi diesels. A typical case: truck comes in with hard cold starts, lots of white smoke, and a rough idle that clears up as it warms. Scan shows P0370 and sometimes a few companion codes. A quick visual inspection reveals a brittle harness going to the pump’s optical sensor, with green corrosion inside the connector. After repairing the wiring and clearing the code, the truck starts clean and the owner notices better throttle response and fuel economy.

Symptoms of P0370

• Hard starting – especially when the engine is cold or after sitting overnight.
• Rough idle – unstable idle speed, shaking, or intermittent stalling at stoplights.
• Loss of power – sluggish acceleration, poor towing performance, or reduced top speed.
• Excessive smoke – white, gray, or black exhaust smoke due to incorrect injection timing.
• Misfires – engine feels like it’s skipping or stumbling under load.
• Poor fuel economy – more frequent fill-ups as the ECU struggles to control timing.
• No-start condition – in severe cases, the engine may crank but never fire.
• Check engine light – MIL illuminated, sometimes with additional timing or sensor codes.

Common Causes of P0370

Most Common Causes

• Failed optical / timing sensor – On many diesels, an internal pump optical sensor or high-resolution crank sensor fails and sends erratic signals.
• Damaged wiring harness – Cracked insulation, chafed wires, or rodent damage near the pump or crank sensor causing intermittent signal loss.
• Corroded connectors – Moisture intrusion at the sensor connector or ECU plug leading to high resistance and signal distortion.
• Incorrect air gap or sensor alignment – On some crank sensors, improper mounting or debris between sensor and tone ring affects signal quality.
• Internal injection pump wear – On older pumps, internal wear can affect the optical wheel or reference disc, confusing the sensor.

Less Common Causes

• ECU (PCM) failure – Rare, but a failing engine computer can misinterpret a good signal as faulty.
• Poor grounds or power supply – Weak battery, bad engine ground strap, or low system voltage affecting sensor output.
• Aftermarket tuning issues – Aggressive tunes or poorly written calibrations upsetting timing reference parameters.
• Mechanical timing problems – Slipped timing chain/belt, worn keyway, or damaged tone ring causing the sensor to read out of phase.
• Previous repair errors – Wrong sensor part number, incorrect installation, or pinched harness during other engine work.

Diagnosis: Step-by-Step Guide

To properly diagnose P0370, you’ll want a basic scan tool (preferably one that can read live data and Mode $06), a digital multimeter, and access to wiring diagrams. A lab scope is ideal for checking sensor signals but not strictly required for a DIY check.

1. Confirm the code and check for companions. Scan the ECU for all stored and pending codes. Note any related crank, cam, or injection pump codes; they help narrow the fault.
2. Check freeze-frame data. Look at engine RPM, coolant temp, and vehicle speed when P0370 set. If it triggers mainly during cold start or high RPM, that’s a valuable clue.
3. Perform a visual inspection. Inspect the harness and connectors to the optical/crank sensor and injection pump. Look for oil contamination, broken clips, rubbed-through insulation, or loose plugs.
4. Test battery and charging system. Low voltage can skew sensor signals. Verify battery condition and alternator output before chasing more complex issues.
5. Check sensor power and ground. With a multimeter, confirm that the timing sensor has proper 5V or 12V reference (per the service manual) and a solid ground. Any drop or fluctuation can cause P0370.
6. Measure sensor signal. If possible, use a scope or scan tool live data to view the RPM or timing reference signal while cranking and idling. Look for dropouts, noise, or erratic readings.
7. Wiggle-test the harness. With the engine running (or cranking), gently move the harness and connectors near the sensor and pump. If the engine stumbles or the RPM reading jumps, you likely have an intermittent wiring issue.
8. Inspect mechanical timing. If wiring and sensor check out, verify that the crank pulley, tone ring, and timing components are tight and properly aligned. On belt/chain engines, look for signs of stretch or slip.
9. Compare with known-good data. If you have access to service information, compare your live data (RPM, timing advance) with known-good values. Large deviations may point to pump or internal engine issues.
10. Clear codes and road-test. After any repair or adjustment, clear the code and drive under similar conditions to the freeze-frame. Monitor live data to confirm the signal is now stable.

Pro tip: If you’re replacing a timing or optical sensor, always clean the mounting surface and inspect the tone wheel or optical disc for cracks, chips, or contamination. Installing a new sensor on a damaged wheel can bring the code right back.

Possible Fixes & Repair Costs

Repairs for P0370 range from simple wiring fixes to major component replacement. Common solutions include repairing or replacing damaged wiring and connectors, installing a new optical or crankshaft position sensor, cleaning or replacing a contaminated optical window in the pump, or addressing mechanical timing issues. In more severe cases, an injection pump rebuild or replacement may be required.

Typical repair costs vary widely: a basic wiring repair might run $100–$250, a new crank or optical sensor $150–$450 parts and labor, and an injection pump job can easily reach $800–$2,000+ depending on the vehicle and labor rates. Factors that affect cost include access to the sensor or pump, whether additional timing components are worn, and if any ECU reprogramming is needed after repairs.

Can I Still Drive With P0370?

In many cases, you can still drive with P0370, but it’s not ideal. If the engine starts and runs reasonably well, you might get by for a short time, especially for local trips. However, you risk sudden no-starts, poor throttle response, and increased smoke. If the engine runs very rough, stalls, or struggles to start, you should avoid driving and have it towed. Continuing to drive with unstable timing can increase stress on internal components and may damage the catalytic converter or diesel aftertreatment system.

What Happens If You Ignore P0370?

Ignoring P0370 can turn a relatively simple electrical or sensor issue into a much bigger problem. Over time, incorrect injection timing can cause excessive cylinder pressures, higher combustion temperatures, and carbon buildup. You may see worsening fuel economy, more smoke, and eventually a no-start condition that leaves you stranded. On diesels with emissions systems, prolonged mis-timing can shorten the life of the catalytic converter, DPF, or EGR components, adding significant repair costs.

Last updated: March 1, 2026

Vehicles Commonly Affected by P0370

P0370 shows up most often on diesel-powered vehicles that rely on an optical or high-resolution crank sensor for injection timing. You’ll commonly see it on:

• GM trucks and SUVs – especially older 6.5L diesel engines (Chevy and GMC pickups, Suburban, Tahoe, etc.).
• Ford diesel trucks – certain Power Stroke generations where crank/cam timing signals are critical to injection control.
• Volkswagen and Audi diesels – TDI engines with rotary injection pumps and internal timing sensors.
• Mercedes-Benz and other European diesels – models using mechanically driven pumps with electronic timing feedback.
• Some older gasoline engines – vehicles with distributorless ignition systems that still use high-resolution timing sensors.

Modern common-rail diesels are less prone to P0370 specifically, but similar timing and crank/cam sensor codes can appear when their sensors or wiring fail.