Genuine Cummins Onan spark arrestor muffler for compatible Onan RV generator exhaust systems. Replaces Onan 155-1222 and helps restore proper exhaust flow, sound control, and safe generator operation.
The muffler is a critical part of an RV generator exhaust system. It helps reduce exhaust noise, route exhaust gases safely away from the generator compartment, and support proper generator performance. Over time, an original muffler can rust, crack, develop pinholes, loosen at the seams, or become damaged from road debris and vibration.
The Cummins Onan 155-1258 is a replacement generator exhaust muffler for compatible Onan RV generator applications. It is commonly used as the replacement for Onan 155-1222 and is built to match the exhaust layout, inlet size, outlet size, and body dimensions required for compatible installations. Because RV generator exhaust systems vary by generator model, chassis, and coach builder, always compare the existing muffler and generator information before ordering.
OEM Cummins Onan replacement muffler designed for compatible Onan RV generator exhaust systems
Common replacement for earlier Onan muffler part number 155-1222
Restores exhaust sound control when the original muffler is rusted, cracked, or leaking
Listed by Cummins as an Onan spark arrestor muffler for generator exhaust use
This muffler is not a universal exhaust part for every Onan generator. Before ordering, compare your existing muffler dimensions, inlet/outlet size, generator model/spec, and tailpipe routing. A muffler that looks close may still be incorrect if the inlet size, outlet size, body length, hanger position, or outlet angle does not match the original installation. If your RV has a custom exhaust extension, modified tailpipe, rusted hanger, or missing hardware, additional exhaust parts may be required for proper installation.
Before Starting: Allow the generator and exhaust system to cool completely. Disconnect generator power as needed and follow all RV and generator service precautions. Basic Replacement Steps: 1. Confirm the Existing Muffler: Compare the original muffler body length, inlet size, outlet size, and tailpipe routing against the replacement muffler. 2. Inspect the Exhaust System: Check clamps, hangers, brackets, tailpipe extensions, and mounting points for corrosion or damage. 3. Remove Old Hardware: Loosen or cut away rusted clamps and hardware as needed. Support the muffler and tailpipe before removing final fasteners. 4. Remove the Old Muffler: Carefully separate the muffler from the inlet pipe and outlet/tailpipe connection. Penetrating oil may be needed on rusted joints. 5. Test Fit the New Muffler: Confirm alignment before tightening. The muffler should sit without strain on the inlet pipe, outlet pipe, or hanger points. 6. Install Clamps and Supports: Reuse serviceable hardware or install new clamps/hangers as required by the exhaust setup. 7. Check Clearance: Verify proper clearance from fuel lines, wiring, body panels, insulation, and combustible materials. 8. Test Run Generator: Start the generator and check for exhaust leaks, rattles, abnormal vibration, or contact with nearby components.
Package Contents: • (1) Cummins Onan 155-1258 RV generator muffler NOT Included — May Also Need: • Exhaust clamps • Tailpipe or exhaust extension • Hanger brackets or mounting straps • Exhaust gasket or sealing hardware, if required by application • Replacement bolts, nuts, or washers • Heat shield or exhaust wrap, if used in the original installation Existing hardware may be rusted or damaged during removal, so inspect the full exhaust system before beginning the repair.
To confirm compatibility before ordering, have the following ready: • Generator model number • Spec letter • Serial number • Existing muffler part number, if visible • Muffler body length, inlet size, outlet size, and tailpipe routing Why this matters: RV generator exhaust systems can vary by generator series and RV manufacturer installation. Confirming the generator nameplate and exhaust dimensions helps avoid ordering a muffler that is close, but not correct for the coach.
Always verify generator model information and physical exhaust dimensions before ordering. RV generator mufflers may look similar while using different inlet sizes, outlet sizes, pipe routing, or mounting arrangements.
Exhaust leaks should be repaired promptly. Generator exhaust contains carbon monoxide, which can be dangerous if exhaust gases enter the RV interior or collect near windows, doors, compartments, or slide-out seals.