Timing Reference High Resolution Signal A No Pulses


The PCM has detected an abnormality in the high resolution timing reference signal. The “A” pertains more to an area of the system circuitry than to a particular component.

Code Set Parameters

If the PCM detects variations in the high resolution engine timing signal that exceed a predetermined number of degrees, a code will be stored and a service engine soon lamp may be illuminated. The maximum allowable number of degrees of variation may differ between manufacturers, vehicle lines, engine sizes/types, and transmission applications.


Symptoms can vary between those that are not detectable at all to engine misfire and spark knock upon acceleration. Fuel mileage may also suffer as a result of the possible conditions associated with this code. Despite the fact that a code is usually stored on the initial failure cycle of these types of failures, malfunction lamp illumination may require multiple failure cycles on certain models.

Common Causes

Professional technicians report that the most common causes of this type of code include defective camshaft, crankshaft, and distributor position sensors. Following closely behind is mechanical distributor failure, which is common in high-mileage rear-wheel drive domestic applications. Faulty, damaged, or corroded wiring and electrical connectors are also contributed with failures of a similar nature. PCM failure is possible but rarely occurs.

Common Misdiagnosis

Unnecessary replacement of crankshaft, camshaft, and distributor sensors, when mechanical distributor failure is the culprit, are fairly common. Crank and cam reluctor rings should also be checked when replacing sensors (where applicable). When replacing crankshaft or camshaft sensors, it is recommended that you do so as a set. The use of OEM sensors is also recommended.


  • Manufacturers utilize the electro-magnetic crankshaft position sensor in two different manners
  • Both designs use either a reluctor ring or teeth (attached to the crankshaft) to interrupt the field of the stationary electro-magnetic CSP sensor; creating what is interpreted by the PCM as a square wave form pattern
  • These interruptions provide the PCM with the precise crankshaft position
  • In the first design, the PCM uses the crankshaft position only for misfire detection and is not critical to spark timing or ignition
  • Engine control systems that use this type of system may allow the engine to start and run, despite a crankshaft position circuit failure, but engine performance and fuel economy will likely suffer
  • Malfunction indicator lamp illumination may not occur until multiple failures are documented in this type of system
  • In the second type of system the PCM uses crankshaft position to calculate spark timing and ignition control
  • A crankshaft position sensor failure in this system design will normally lead to a no-start condition, an immediate stored trouble code, and an illuminated service engine soon lamp (on the first failure). A viable starting point for obtaining a successful diagnosis of the crankshaft position sensor is by checking for an engine RPM signal when the engine is cranked or running
  • This is accomplished by using a scanner or observing the vehicle tachometer as the engine is being cranked (using the starter) or is running, depending upon CPS system design
  • If no RPM signal is detected, then visually inspect the crankshaft gear, crankshaft position sensor, and sensor connector for damage and repair as necessary
  • If no obvious signs of damage are found, then test the CPS system for a reference voltage signal (typically 5-volts, but check manufacturer’s specifications)
  • If you have access to an oscilloscope, then test the CPS signal wire for the presence of a square 5-volt waveform pattern for each engine revolution
  • If no pattern is detected then test the resistance of the (disconnected) CSP sensor and compare the values with manufacturer’s specifications
  • If the CSP sensor checks out, then test the system circuitry for the proper voltage and resistance
  • Repair open or shorted wiring as necessary
  • While PCM failure is certainly a possibility, it is rare and all other possibilities should be exhausted prior to condemning the PCM. The camshaft position sensor and distributor hall-effect sensor are used in a similar manner to the crankshaft position sensor
  • Inputs between the three sensors help to control ignition timing and fuel delivery. The camshaft position sensor is an electro-magnetic sensor that interacts with a metal reluctor ring (or gear) on one end or the other of the camshaft
  • Engines that utilize multiple camshafts (dual-overhead cam engines) are equipped with multiple camshaft position sensors
  • As the reluctor passes by the sensor, a precisely placed hole or gap in the teeth interrupts the waveform pattern sent by the sensor to the PCM
  • This interruption correlates with an ignition timing reference value that is programmed into the PCM
  • Variations from the manufacturer’s reference timing value (seen as voltage waveforms) will cause a code to be stored and possibly a malfunction indicator lamp to be illuminated. Several specialty tools will be required to diagnose this code successfully
  • They include a scanner, a digital volt/ohmmeter, and possibly an oscilloscope. Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • In the event of an intermittent condition, you may also utilize the oscilloscope to monitor waveforms created by the distributor, camshaft, and/or crankshaft sensor/s, while looking for glitches or other inconsistencies. Suspect areas of system circuitry that are contaminated with oil, antifreeze, or power steering fluid that has leaked from the engine
  • If wiring with missing or distorted insulation is found, repair or replace it as necessary. If no obvious system circuitry problems are discovered, perform a resistance test at the crankshaft, camshaft, and distributor position sensors and a voltage test on the sensor connector
  • Using your digital volt/ohmmeter test reference voltage at the sensors and compare your findings with the manufacturer’s specified reference voltage
  • If system reference voltage readings are in line with specified values (or if sensor resistance values do not coincide), replace the camshaft sensor, distributor hall-effect sensor, and the crankshaft sensor. Inspect the distributor (where applicable) for excessive end-play and side-to-side wobble
  • If the distributor is sloppy or worn out, replace it and retest the system for failures. If system voltage readings do not coincide with manufacturer’s specified reference figures, check system continuity using your digital volt/ohmmeter
  • Use caution when checking resistance values in wiring that is connected to the PCM
  • For best results, disconnect the electrical connector from the PCM prior to using an ohmmeter on the harness side of the circuit
  • Remember that PCM failure is possible but very rare