CAT 49 Fault Code: Inlet Air Heater current low
Also called Inlet Air Heater current high
Inlet Air Heater current low · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 49 (SPN 729 / FMI 6) means the ECM detected either a high current (short circuit) or low current (open circuit) condition while trying to run the air inlet heater. It's an electrical circuit fault in the heater, relay, or wiring, not a heater performance complaint. The engine will likely still start, but cold-start smoke and cranking may get worse.
Medium severity. The engine can still be started and driven, but cold-weather starting will suffer and white smoke at startup may increase. It should be diagnosed before cold weather sets in, not ignored long-term.
What does CAT error code 49 mean?
CAT code 49 covers two related fault conditions on the same circuit: an inlet air heater current high fault (short circuit) and a current low fault (open circuit). Both are reported under SPN 729 / FMI 6, and both point to the electrical circuit that drives the intake air heater, not a mechanical or thermal problem with the manifold itself.
The air inlet heater warms incoming air so a cold engine starts easier and produces less white smoke at startup. The ECM controls the heater and its indicator lamp through a relay, and it runs the heater on a set cycle tied to engine power-up, cranking, and normal running, adjusted by coolant temperature, intake air temperature, vehicle speed, and brake pedal position.
When the ECM commands the heater on and sees current draw that's too high (a short) or too low (an open circuit) for longer than its set time window, it logs code 49. This tells you the heater circuit itself, not the strategy that decides when to run it, has a wiring or component problem.
What triggers a CAT 49 code?
For the high current (short circuit) version: the engine must not be cranking, the ECM must have been powered for at least 1 second, battery voltage must be above 9 V for more than 2 seconds before the fault, and the high current condition must persist for more than 2 seconds. For the low current (open circuit) version: the engine must not be cranking, the ECM must have been powered for more than 1 second, battery voltage must be above 9 V for more than 2 seconds before the fault, and the low current condition must persist for more than 8 seconds.
Common causes of 49
- ECM connection: a loose, corroded, or damaged connection at the ECM affecting the air inlet heater control circuit.
- Air inlet heater relay: a failed or sticking relay that fails to switch heater current correctly, causing either no current flow or excessive current flow.
- Air inlet heater: the heater element itself may be shorted internally or open (burned out or broken).
- Air inlet heater wiring connection: damaged, corroded, or disconnected wiring/connectors between the ECM, relay, and heater.
How to troubleshoot CAT 49: first checks
- Inspect the air inlet heater wiring harness and connectors for corrosion, looseness, chafing, or damage between the ECM, relay, and heater.
- Check the air inlet heater relay for proper operation, listen/feel for it clicking when the heater cycle should activate, and inspect its connector and mounting.
- Verify battery voltage is solid and above 9 V during key-on, since the ECM won't log a valid heater fault reading if supply voltage is out of range beforehand.
- Check the ECM connector itself for corrosion or a poor pin connection on the circuits feeding the air inlet heater control.
- With the engine not cranking, confirm whether the heater lamp behaves as expected on power-up, since it should illuminate for a minimum of 2 seconds at ECM power-up regardless of coolant temperature; incorrect lamp behavior can help confirm a circuit fault versus a control strategy issue.
- Inspect the heater element itself for continuity and obvious physical damage (burned, cracked, or broken element) if wiring and relay check out fine.
How the code clears
No separate clearing step is listed for this code beyond repairing the underlying wiring, relay, or heater fault. Once the current draw returns to a normal range during a heater activation cycle that meets the ECM's timing conditions, the fault should stop being active. If a stored code remains after the repair, follow your normal procedure for clearing stored diagnostic codes on the ECM.
Frequently asked questions
What does CAT code 49 mean?
It means the ECM tried to turn on the air inlet heater and detected either too much current (a short circuit) or too little current (an open circuit) in that circuit. It's logged as SPN 729 / FMI 6.
Can I still drive with CAT code 49 active?
Usually yes. The air inlet heater affects cold starting and startup smoke, not engine operation once running. But cold-weather starts may become harder and smokier until it's fixed.
Is code 49 a relay problem or a wiring problem?
It can be either, or it can be the heater element itself or a bad ECM connection. CAT lists all four (ECM connection, relay, heater element, wiring connection) as probable causes, so each needs to be checked.
Why does vehicle speed matter for the air inlet heater?
The ECM turns the heater off once the vehicle reaches or exceeds 8 km/h (5 mph) with the brake pedal depressed. If the vehicle speed sensor has an active fault, the ECM assumes speed is above that threshold and disables the heater when the brakes are applied, which can affect diagnosis timing.
How long is the air inlet heater supposed to run at startup?
At altitudes below 1678 m (5500 ft), the ECM runs the heater for 30 seconds if the combined coolant and intake air temperature sum is less than 40 °C (136 °F). Above 1678 m (5500 ft), that same 30-second cycle runs if the sum is less than 53 °C (160 °F).
Does the heater lamp always turn on at startup?
Yes. CAT specifies the lamp should turn on for a minimum of 2 seconds at ECM power-up regardless of coolant temperature. If it doesn't, that can point toward a lamp or relay circuit issue related to code 49.
What's the difference between the high current and low current versions of code 49?
High current means a short circuit was detected for more than 2 seconds after battery voltage was confirmed above 9 V for 2 seconds. Low current means an open circuit was detected for more than 8 seconds under the same voltage condition. Both point to the same heater circuit but different failure types.