P1303 – Cylinder 3 Misfire (Saab)

By Reinier (Contact Me)
Last Updated 2022-02-07
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1303

What Does Code P1303 Mean?

OBD II fault code P1303 is a manufacturer-specific trouble code that is defined by carmaker Saab as, “Cylinder Number 3 Misfire Detected”, and is set when the PCM (Powertrain Control Module) detects a persistent or cyclical misfire in cylinder #3.

NOTE: Persistent or cyclical misfires in any one or more cylinders are misfires that occur at every engine cycle i.e., the air/fuel mixture fails to ignite, or ignites only partially at the end of every compression stroke. Random or non-cyclical misfires on the other hand, are misfires that occur randomly, and often under different operating conditions and under varying environmental conditions, which can make it difficult to replicate and/or diagnose the root cause of the problem.

As with most other vehicle makes, Saab vehicles use the crankshaft position sensor to detect misfires, and in the broader context of the engine management system, this application of the crankshaft position sensor forms the basis of the misfire detection system. Here is how the misfire detection system on Saab vehicles works in practice-

The crankshaft position sensor works in conjunction with a reluctor wheel on the harmonic balancer (crankshaft pulley) or on the crankshaft inside the engine block, or sometimes, on some Saab models on the flywheel, to generate electrical signals as the teeth on the reluctor ring’s teeth move past the sensor’s sensing element. We need not delve into the different types of crankshaft sensors in use on Saab vehicles here, beyond saying that the electrical signals serve two purposes. The first is that the CPM uses the signals as triggers to initiate ignition sparks, and the second is that a fixed reference point on the reluctor ring is used to calculate the position of piston #1 during cranking before the engine starts.

The reference point produces a slightly different signal, which the PCM interprets as the position of cylinder #1 during both engine cranking, and normal engine operation, and it is this reference point the PCM uses to identify misfires on one or more cylinders.

In practice, the crankshaft rotates at a reasonably constant rate on an engine that is not misfiring, but if a misfire occurs, the misfire produces small variations in the crankshaft’s rotational speed during each revolution of the crankshaft. Note, though, that as a general rule, the PCM ignores speed variations that are smaller than about 2 percent of the crankshaft’s average rotational speed, since all crankshafts undergo small deformations and vibrations during normal engine operation.

As a practical matter, all misfire detection systems use complex algorithms that are based on the number of teeth of the reluctor ring, and the more teeth a reluctor ring has, the higher the resolution of the ignition timing calculation becomes, which greatly increases the accuracy of the misfire detection system.

Nonetheless, if the misfire detection system detects a variation in the crankshaft’s rotational speed that exceeds a maximum allowable threshold, it “notes” the position of the variation (measured in degrees of crankshaft rotation) relative to the fixed reference point on the reluctor ring. This position forms the basis of the calculations the PCM performs to identify the misfiring cylinder, which in the case of OBD II code P1302 on Saab vehicles, is cylinder #3.

Note that in case of persistent, cyclical misfires, the PCM will typically disable the fuel injector on the affected cylinder to protect the catalytic converter(s). Thus, when the PCM detects a random misfire on cylinder #3 (or on any other cylinder(s)), it will set code P1303, and illuminate a warning light after a pre-defined number of ignition failures or misfires had occurred.

NOTE: In the case of random misfires, the PCM may not always disable the injector on the affected cylinder when a misfire occurs, which could eventually damage or even destroy the catalytic converter(s) if the fault persists for extended periods.

Where is the P1303 sensor located?

This image shows the location (arrowed) of the crankshaft position sensor on a 2005 Saab 2005 2.2 TiD application. Note that while the crankshaft position sensor is relatively easy to access on this vehicle, it may be difficult to access the sensor on some Saab applications.

In some cases, it may be necessary to remove and/or disassemble unrelated parts and/or components such as the drive belt, alternator, radiator, coolant hoses, HVAC lines, power steering lines, and other parts simply to gain sufficient workroom to access the crankshaft position sensor.

Thus, if you suspect that the problem might be caused by a defective crankshaft position sensor and/or a damaged reluctor ring, we recommend that you consult reliable service information for the affected application to prevent damage to any parts and/or components during the disassembly process.

What are the common causes of code P1303?

Cyclical or persistent misfires on Saab applications could have a large number of possible causes, some of which could include one or more of the following-

• Defective or malfunctioning spark plug
• Defective or malfunctioning ignition coil
• Defective or malfunctioning crankshaft position sensor, but note that it is far more likely that a damaged reluctor ring will produce persistent cyclical misfires on only one cylinder
• Loss of compression on cylinder #3 through a damaged or leaking cylinder head gasket, but note that if the leak path is not between two adjacent cylinders, the leak path is between the affected cylinder bore and either the water jacket, or an oil gallery
• Damaged or leaking valve seat insert
• Defects in and malfunctions of the primary ignition circuit of cylinder #3; such defects include burnt, shorted, damaged, and corroded wiring and /or electrical connectors
• Defective or malfunctioning fuel injector on cylinder #3
• Defects in the injector’s wiring, with possible defects including burnt, shorted, damaged, and corroded wiring and /or electrical connectors
• Engine vacuum leaks, such as small localized failures of intake manifold gaskets and/or seals that only affect cylinder #3
• Mechanical issues, such as sticking valves, broken rocker arms, broken valve springs, or worn camshaft lobes might cause misfires to occur on affected cylinders
• PCM issues, such as malfunctions of the ignition driver for cylinder #3
• Defective or corrupted software in the PCM, but note that this will rarely affect only one cylinder. However, since there are exceptions to this rule, the fault must first be sought elsewhere before any control module is reprogrammed or replaced

What are the symptoms of code P1303?

Common symptoms of code P1303 on Saab vehicles could include one or more of the following, but note that depending on the engine speed/load, the severity of some symptoms may vary between different engine designs-

• Stored trouble code and possibly an illuminated warning light
• Depending on the nature of the problem, one or more additional codes may be present along with P1303
• The engine will have significantly reduced power
• The sparkplug on cylinder #3 may be fouled with carbon
• It may be difficult to start the engine both when it is hot and cold
• Fuel consumption may increase
• The engine may stumble or hesitate momentarily under acceleration
• Depending on the nature of the problem, the engine may overheat
• There may be excessive pressure in the cooling system
• Depending on the nature and severity of the problem, the engine may run a) roughly at idling speed, or run roughly at all, or most engine speeds and loads
• In some cases, and depending on both the engine design and the engine speed/load, the misfire may disappear, or the misfire may not be apparent at high engine speeds and/or high engine loads
• In some cases, catalytic converter damage could result if the problem is not found and corrected promptly
• In rare cases, and depending on both the affected model and the nature of the problem, a persistent misfire on any cylinder could produce harsh gearshifts under aggressive acceleration, or at least, as aggressive as a misfire will allow

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