Isuzu I-Mark Back Up Light Bulb Size

The Isuzu I-Mark back up light utilizes bulb size 1073 and bulb size P21W for illumination. Available data covers the range from 1981 through 1989 for the specified vehicle.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1989	1073
1981-1988	P21W

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. Lower wattage variant of 1156. 12V, 13W.	1073
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The Isuzu I-Mark back up light specifications cover model years 1981 through 1989.
Two bulb types appear across these model years: the 1073, an ANSI-standard bulb used in North America and Canada, and the P21W, an ECE-standard bulb used in Europe and worldwide markets.
Both the 1073 and P21W share the BA15s bayonet base, a single-contact base type that uses a 15mm barrel with a single locating pin on the side.
Both bulbs are single-filament types, meaning each is designed for one lighting function rather than serving dual roles such as combined stop and tail operation.
Both bulbs operate at 12 volts, consistent with standard automotive electrical systems.
The P21W operates at 21 watts, while the 1073 operates at 13 watts, making the 1073 a lower-output bulb despite sharing the same base type.
The 1073 is described as a lower wattage variant of the 1156, a common ANSI bulb that also uses the BA15s base at 12V but draws approximately 26.9 watts.
The Isuzu I-Mark (1989) is the only model year in these specifications fitted with the 1073 bulb, while the 1981-1988 model years are specified with the P21W.
Because the 1073 and P21W share the BA15s base, physical installation is mechanically compatible between the two types, though their wattage ratings differ and substitution may affect circuit performance or regulatory compliance depending on the market.
The shift from P21W to the 1073 in the 1989 model year reflects a change in the applicable lighting standard, from ECE to ANSI, for that production year of the I-Mark.