SPN 729 Howo — intake-heater relay circuit

Fault codes / SPN 729

Without FMI this is not a diagnosis. SAE/Cummins SPN 729 is Engine Intake Air Heater Driver #1. FMI 3 high/short to power, FMI 4 low/short to ground, FMI 5 open / no relay (Howo field tables). This is the live circuit, not config SPN 2898. No OEM in the card.

Which trucks see it

Howo/Weichai WP with a grid, some Cummins ISM on Shacman (FC 2555/2556). Relay often on the frame or by the ECM.

Symptoms

• hard winter start
• MIL with no derate when warm
• relay does not click
• 729 or 729/3/4/5 on the scanner

Likely causes

• Driver circuit — FMI picks the branch.
• Relay, fuse, plug, wet after a wash. Grid open/short after the relay.
• Do not confuse with SPN 2898.

What to check

1. Read FMI.
2. Key on cold: relay click, voltage on the grid, heat on the pipe.
3. Fuse, pins, coil resistance. If 2898 is also bare, open that card.

Can you keep driving

You can drive. In frost do not soak it until the relay/grid is checked. Home form: 729, 729/5 or a photo.

Look up the code in the service

Other codes

• P0087 — low common-rail pressure
• P0088 — high common-rail pressure
• P1011 — HP pump metering valve fault
• P0093 — large fuel leak / CAN trap — FMI needed
• P0193 — rail pressure sensor high signal
• P0299 — boost below target
• P2263 — turbo / boost performance
• P2563 — VGT position sensor — range / performance
• P0335 — crankshaft sensor no signal
• P0340 — camshaft sensor (CMP) — FMI needed
• P0401 — EGR insufficient flow
• U0100 — lost communication with engine ECU
• U0101 — lost communication with TCM
• U0155 — lost communication with instrument cluster
• P0562 — system voltage low
• P0201 — cylinder 1 injector circuit
• P0480 — fan control circuit
• P0217 — engine overheat
• P0116 — water in fuel (Weichai table) / not always coolant SAE
• P2509 — intermittent ECM power (no OEM in DB)
• SPN 2898 — without FMI — not a diagnosis (config / Weichai grid)